Nanite

Nanites are microscopic machines which can enter a person's body. They can be used for a variety of uses.

Ancients
During the war with the Wraith, the Ancients used nanite technology in an attempt to destroy the Wraith from the inside. They gave the nanites the ability to replicate, and installed an aggression code surpassing that of the Wraith. To protect themselves, the Ancients installed a law into their base code that prevented them from harming anyone with the ATA Gene. However, the nanites instead took human form, becoming the Asurans. The Ancients eventually concluded that the Asurans would never become the weapon they wanted, and tried to destroy them. However, their efforts failed.

Goa'uld
Seeking to learn what humans would evolve into, the Goa'uld Pelops used nanites to accelerate the human lifespan to simply one hundred days. On the planet Argos, Pelops confined the humans to a small area, as the nanites needed to be within reach of a device which would send out instructions to the nanites, causing them to accelerate the life-cycle while the Argosians were sleeping. Jack O'Neill was infected with the nanites, causing him to age to 100 in a matter of weeks. However, after the Argosians destroyed the statue of Pelops containing the device, the nanites were destroyed by the immune systems of the humans, allowing them to age at a normal rate.

Orbanians
The Orbanians used nanite technology as a means of educating their people. Certain children, designated Urrone would be selected to have their nanites removed from their body, allowing the rest of the Orbanians to learn what they had learned. However, the removal process rendered the child mentally an invalid. Attempts to reintroduce the nanites failed, and the children were taken to secure facilities where they were well cared for, but no efforts were made to re teach them. Eventually, after learning that their was more to life than learning, the Urrome Merrin allowed her nanites to be removed, which also implanted a part of her personality on the Orbanians, who then decided to teach other Urrone the "old fashioned way".

Replicators
When the Replicators arrived on Halla and found Reese, they hailed her as their creator, and found her nanite cells possessed certain advantages that regular Replicator blocks did not, including resistance to Tau'ri guns. The Replicators created five Human Form Replicators. These Replicators could probe the minds of individuals and learn vital information. Replicator nanites require neutronium to function.