Subspace communication is a method of communication between two points by sending the information through subspace.
Communication by culturesEdit
The Ancients developed subspace communicators which are installed on all their known spaceships. They also developed a long-range communication device capable of transmitting large amounts of data at speeds allowing real-time communication even over billions of light-years. They also posess palm-sized devices which can transmit a similar amount of information between at least distant planetary systems within a galaxy. (SG1: "Avalon, Part 2") (SGA: "The Defiant One")
The Asgard use subspace communication devices on their ships and installations which facilitate real-time communication at least between close galaxies, but are not likely to reach as far as the Ancient long-range communication devices. Holograms can be transmitted over said link, as well as other forms of data. (SG1: "Thor's Chariot", "Red Sky", "Fail Safe", "Unnatural Selection")
The Goa'uld used long range visual communication devices to communicate between stars. It allowed them to communicate with each other visually. (SG1: "Within the Serpent's Grasp") Their ships and probes are also equipped with subspace communication, Yu's First Prime Oshu and Teal'c using it to contact Ba'al. (SG1: "Homecoming")
The Replicators were linked together through a subspace communication network, which is what allowed them to store information so quickly. It also allowed them to upload "immunities" to weapons that they encountered. (SG1: "New Order, Part 2", "Gemini")
Given the Asgard's gifting of their technology to the Tau'ri, it's reasonable to assume the Tau'ri possess the Asgard's most recent subspace communication technology, if they haven't developed their own. Either way, they are in possession of such capabilities as Earth ships often communicate via video transmission with Earth and Atlantis. Furthermore, Eli Wallace is seen communicating with his mother Marian while en route to P4X-351 in hyperspace aboard the Earth ship George Hammond. His mom was able to receive Eli's call over a conventional telephone, which suggests that the Tau'ri at least have the ability route voice calls placed to Earth via subspace through terrestrial and wireless switchboards to civilian phones. Given the vast distances often between parties, the only viable real-time communication would be transmitted through subspace.
The Tok'ra use specially-designed communicators able to work through their tunnels system and dirt. The Tok'ra also use subspace beacons to track Goa'uld ships. Unlike the Goa'uld version, these are more than compatible with the Ancient version, but still require a stone to communicate with. (SG1: "Last Stand", "Reckoning, Part 1", "Crusade")