In the fictional universe of the Stargate? franchise, the people of Earth have encountered numerous extraterrestrial races on their travels through the Stargate. In addition to a diversity of alien life, there is also an abundance of other humans, scattered across the cosmos by advanced aliens in the distant past. Some of the most significant species in Stargate SG-1 are the Goa'uld, the Asgard, and the Replicators.
Stargate Atlantis?, set in the Pegasus galaxy, introduced the Wraith and the Asurans. One of the most influential species in Stargate, the Ancients, have moved on to a higher plane of existence.
Stargate SG-1? explains the human population in the Milky Way galaxy by revealing that the alien Goa'uld transplanted humans from Earth to other planets for slave labor. Many of these populations were subsequently abandoned, often when deposits of the precious fictional mineral naqahdah were exhausted, and developed into their own unique societies.
Some of these extraterrestrial human civilizations have become much more technologically advanced than Earth, the in-show rationale being that they never suffered the setback of the Dark Ages. The most advanced of these humans were the Tollan, although they were destroyed by the Goa'uld in Season 5's Between Two Fires.
The human populations of the Pegasus galaxy are the product of Ancient seeding. few human races in Pegasus are technologically advanced, as the Wraith destroy any civilization that could potentially pose a threat.
There are also large numbers of humans in the Ori galaxy, where they empower the Ori through worship.