The Milky Way is home to many sentient species. A few of them have achieved a high level of technological advancement, while others remain primitive. While the races of other galaxies have influenced the Milky Way a great deal, (such as the Ancients, Asgard, and Replicators,) by far the most influential native species in the galaxy's history have been the Goa'uldSystem Lords.
The A't'trr were once a race not unlike the Reetou. In fact, some believe that the species are related. The A't'trr had a technologically advanced society, but they never deciphered their stargate, and their world had no naquadah, so they never developed hyperdrives. When the orbit of their moon deteriorated and their planet's doom was on the horizon, they sent sleeper ships to nearby worlds, one of which contained the ancestors of the Reetou. They made an advanced time capsule including advanced, sentient microorganisms which was encountered by the SGC.
The Entity was an intelligent computer being whose home-world was more technologically advanced than Earth. The creature was sent to Earth as a weapon against the SGC, because the radio waves of the exploratory MALP were somehow toxic to the inhabitants, and they viewed the MALP's incursion as an act of war.
The Eurondans lived and reproduced based entirely on a concept of genetic purity. They grew disgusted with another nation on their world, which they referred to as "Breeders" due to the fact that they reproduced indiscriminately, resulting in a more ethnically diverse population. They made plans to eradicate the Breeders by poisoning the atmosphere of the planet, making it uninhabitable. The Breeders discovered this, and took preemptive action against their neighbors. After they were about to fall, they contacted the Tau'ri by Stargate, for their urgent need for help. When they learned that the Eurondans started the war, they tried to stop them, realizing they were helping the enemy. SG-1 assisted the Breeder bombers in destroying the facility. The Eurondan facility was heavily damaged, and all apparent information leads to the Breeders retaking complete control of the planet. The Eurondans were likely all killed or captured.
Little is known about the Furlings, except that they were a member of the "Alliance of Four Great Races", along with the Alterans, Asgard, and Nox. However, occasional artifacts attributed to the Furlings have been identified. Record of the Asgard's early campaigns against the Goa'uld indicate that the Furlings were allies in this endeavor, although militarily they were not as mighty as the Asgard. Furling writings have been found in the pyramids of the Omeyocan race; no solid conclusions have been drawn from this fact.
The Gadmeer were an advanced sulfur-based reptile-like species. 1,000 years ago, they were defeated by a superior power and wiped out. They created a giant vessel to travel the stars and find a world that could be repurposed and terraformed into their new homeworld. Due to their unique physiology, this was a long and challenging task. It is completing its task on P5S-381.
The Goa'uld are a race of sentient parasitic beings that take over the bodies of other beings to use as hosts. The hosts available on their native world were the Unas. Other species have also been infrequently used, but once they discovered humanity, humans quickly became their favored hosts. The species is split into two primary factions. The evil “Goa'uld” take hosts against their will and crave power; they vastly outnumber the Tok'ra who coexist with willing hosts only. The evil Goa'uld are arguably the greatest enemy to ever threaten the Milky Way, having held galaxy-spanning empires for thousands of years.
The history of Humanity is more complicated that most modern humans realize. The first evolution of the human form occurred in a distant galaxy, on the planet Celestis. A faction of these humans, known as Alterans, migrated to the Milky Way, and eventually influenced the second evolution of the human form on Earth. When these modern humans were still primitive, the Goa'uld enslaved the planet, taking some humans away from Earth, laying the seeds for thousands of other human cultures to spread throughout the galaxy. As such, many sub-races of humans exist today. Human populations are also present in the Pegasus galaxy.
The Ilempiri were a species that are human in appearance, but with a unique physiology that prevents them from becoming host to Goa'uldsymbiotes. This presented the race with a distinct edge over the Goa'uld. But when the parasitic species learned of it they forced the Ilempiri into subordination, and conceived the drug roshna. The entire race is addicted to the drug, which they must ingest on a regular basis in order to survive.
The Jaffa were once humans, but were genetically modified by the Goa'uld in order to be incubators for their young. This made the Jaffa bound to the will of the Goa'uld, since a Jaffa will die without harboring a Goa'uld. For millennia, the Jaffa were the foot soldiers of the Goa'uld. With the recent defeat of the System Lords, the Free Jaffa Nation has emerged. Despite being utilized by hundreds of different System Lords over a very long period of time, most Jaffa share a surprisingly similar culture.
Netjerians are a tall, thin race with purple-black skin and elongated features. Their homeworld was once claimed as the throne world of the System LordHathor. Since she left the world, the Netjerians have advanced technologically and have become opposed to the Goa'uld.
The Nox are peaceful and fairy-like people, capable of reviving the dead and rendering objects as large as a city invisible. They were a member of the "Alliance of Four Great Races", along with the Alterans, Asgard, and Furlings. They were extreme pacifists who maintained a deep connection to nature and the natural world.
The Ohnes are a long-lived amphibious species. The Goa'uld attempted to invade their world in order to subjugate the populace, and succeeded in causing massive damage, but they were eventually driven off. Ohne freedom fighters are known to work against the Goa'uld on many worlds, and even visited Earth in the distant past.
The Omeyocans are a species which apparently exists in a different phase of visibility that Humans cannot naturally see. To compensate for this, they release massive amounts of neutrinos into the area, allowing humans to view them. In form, they appear as a mist like species. They are enemies of the Goa'uld and seem to have had a presence on Earth in the past. They had some relationship with the Furlings.
The Pod People are a race who, on the outside, appear to be fairly human, but with a unique internal anatomy. The species has no apparent communication other than an unusually pitched "singing" which seems to correlate with the flora around them. They have an extremely symbiotic relationship with a plant-like species on their planet. They seem to be technologically primitive.
The Reol are a peaceful race who were almost wiped out by the Goa'uld. The Reol have a unique natural defense; one of their bodily secretions is used to create false memories and illusions when it comes into contact with a living creature. This is a powerful defense, but has given the Reol the false reputation of being deceitful. The survivors of their race have made a new, hidden colony for themselves.
The Reetou are a mostly peaceful insect-like species from Reetalia (originally, P5C-353) who are phased 180 degrees outside of the human range, rendering them completely invisible to the un-aided eye and mostly immune to projectile weapons in our phase. The Reetou are descended from the A't'trr. The Goa'uld nearly succeeded in wiping out the Re’tu, due to the threat they posed.
The Sekhmet were a bipedal feline species conquered by the Goa’uld and used as hosts prior to the discovery of Earth. After the transition to human hosts, Bastet still utilized the Sekhmet, but eventually she, too, abandoned them. Today, their race seems to exist only in small primitive communities, abandoned and forgotten on a number of worlds.
The Serrakin are a technologically-capable race that is capable of interbreeding with humans. At some point in the past, the Serrakin liberated a population of humans from the Goa'uld and these humans have coexisted with them ever since. Prior to contacting the SGC, the Serrakin could not use their stargate and had only limited space flight capabilities.
The Spirits, as they are known to the SGC, are a technologically advanced race. When the Goa'uld brought human slaves to their world, the Spirits liberated them. Nothing is known about their greater society or politics, or even if PXY-887 is their race's true home; this is because they buried their stargate after contact with the SGC. All that is known is that they have impressive powers, including the ability to shape-shift.
The Stragoth were an aggressive race encountered when they successfully infiltrated the SGC in an attempt to take over Earth. They had powerful mimetic technology and could flawlessly impersonate SG personnel. Apparently, they were attempting to find a new homeworld, suggesting that their race was in some sort of trouble. They had no desire to negotiate or establish dialog.
The Unas are a primitive reptilian race native to the same world as the Goa'uld. They Goa'uld used the Unas as hosts until the discovery of Earth. On their own, the Unas are technologically primitive and their society is tribal, and sometimes brutal.
The Unity are blue crystalline lifeforms. They are essentially energy beings that inhabit crystal containers. They derive nourishment from a planet's electromagnetic field, however, they can only survive in a similar environment. The Unity are capable of manipulating electromagnetic fields and energy to a fine degree. This allows them to reshape their bodies into near perfect imitations of other lifeforms. They are also capable of gaining the memories of those who touch them.
The Water-based life form was a form of intelligent lifeform which resembled water in both its liquid and gaseous form. They lived on a planet which held the ruins of an ancient city, covered in what appeared to be water but which was actually many lifeforms. Their discovery was an accident, and no diplomatic contact has since been made.
The Haaken inhabited the galaxy during the time of the Ancients. In this time, they were noted for being an old and proud race that the Ancients aided by giving them technological secrets. However, the Haaken corrupted these gifts and used them to embark on a reign of terror across known space. A great conflict ensued, resulting in the defeat of the Haaken. Anubis attempted to utilize them as allies.
The Volca were unique in that they had the natural ability to phase shift through solid objects, making them immune to being hosts to Goa'uld. The Goa'uld devastated the Volca's population, and only a few have survived.