Alliance of Four Races
An ancient alliance between the four most powerful species in the Milky Way Galaxy, which came to an end long ago. The four races were the Ancients (who built the Stargate network), the Asgard, the Furlings, and the Nox.
The Alliance met together at the planet nicknamed Heliopolis (after an ancient Earth myth), where early on they developed a means of communication using the most basic language common to everyone in the universe -- molecular depictions of natural elements. The great meeting hall included inscriptions from the language of each of the four races etched into the stone walls.
It is not known just how much the four races shared knowledge and technology, but it is reasonable to assume that the Ancients (who were tens of millions of years old, as a civilization) helped the others along in their own development.
It is also not known how long the Alliance lasted, or what brought it to an end. One likely explanation for its end is the galaxy-wide plague that afflicted the Ancients, prompting them to learn how to ascend, and to immigrate to the Pegasus Galaxy. By the present day, there is no evidence that the Asgard, the Nox, and the Furlings continue to maintain any ties.
The Torment of Tantalus - SG-1 discovers Heliopolis, where four advanced races once met.
Thor's Chariot - Daniel identifies Thor's race, the Asgard, as one of the ancient Alliance members.
The Fifth Race - Jack visits the Asgard in another galaxy, who tell him of the Alliance of four great races: the Asgard, the Nox, the Furlings, and the Ancients.
Unending - With the Asgard civilization on the verge of destruction, Thor tells Samantha Carter that humans have become "the fifth race," reaching the point where they can truly call us equals.