Shra'kt'lor is the traditional capital city of the Githzerai in Limbo, ruled over by their god-king Zaerith Menyar-Ag-Gith. Detailed in the Planes of Chaos boxed set for Planescape, it consists of some 2 million active inhabitants and could support many more: it is intended to serve as the last refuse and final line of defense, should the githzerai's enemies ever invade Limbo and come after them.
As the book notes, such a "last stand" location is not so much a matter of necessity as it is an eloquent statement about the githzerai mindset.
In Planescape: Torment, the githzerai Zerth named Dak'kon was originally a respected denizen of Shra'kt'lor, until the day his crisis of faith caused a fatal weakness in its psionically-stabilized walls and resulted in the city being devastated; another githzerai NPC in the game will clarify that this was a result of a sudden, massive attack by the githyanki.
Whether or not Shra'kt'lor was destroyed by this assault is something that Planescape fans argue about. For what it's worth, it's mentioned in passing in the 3rd edition Manual of the Planes as still existing. In 4th edition, its place is taken by Zerthadlun.
|The Gith of Dungeons & Dragons|
|Races:||Duthka'gith - Gith - Githyanki - Githzerai|
|Undead:||Kr'y'izoth - Tl'a'ikith|
|Individuals:||Vlaakith CLVII - Zaerith Menyar-Ag-Gith - Zerthimon|
|Places:||Shra'kt'lor - Tu'narath|
|Miscellaneous:||Crown of Corruption - Gish - Scepter of Ephelomon|
|Adventures:||Incursion - Scales of War|