The Xaositects, or Chaosmen, are a faction from Planescape. In essence, these barmies believe that there is no meaning to the Multiverse - not that there is no point, like the Bleak Cabal, but rather that things like order, patterns, etc are all just flukes. They preach that Chaos is all that there really is - total blind chance, random luck, pure whimsy. Nothing makes any sense because the Multiverse isn't supposed to make any sense. So, people should just accept that! Do whatever the hell you want; if you feel like doing it, then you should, because there's no reason not to do so.
...If this is ringing alarm bells in your head, then yes: this is basically a philosophy that worships the idea of Chaotic Stupid, and actually considers this the ideal way to behave. Consequently, "I want to play a Xaositect!" is generally met with the same alarm and hostility as the phrase "I want to play a Kender!"
In 2nd edition, belonging to the Xaositects required a Chaotic character alignment, forbade a character from participating in activities that required long-term organization or discipline, made them freakishly adept at knowing instinctively where lost things were, and let them cast Babble (a reversed version of the Wizard spell Tongues) 1/week for basic membership. Those who attained the next rank (Boss) gained Nondection against Divination spells cast by Lawful sources at level 5, and the ability to shoot off a Confusion spell 1/day that hits anyone within 20 feet and lasts for 2D6 rounds; Lawful characters are -2 to their saving throws against this effect. Big Bosses can have all sorts of weird powers, like Wand of Wonder, a permanent Unseen Servant or Alter Self at random.
In 3rd edition, Xaositect was a prestige class issued in Dragon Magazine #287. It required a Chaotic alignment, BAB +4 and +2s in all saves. It has an appropriately random grab-bag assortment of abilities, including protection from divination by Lawful casters, a vocalizing-nixing ability called Babble, dice-rerolling, resistance to Lawful-aligned spells, immunity to Illusion (Pattern) spells, a Confusion aura, a touch attack that compels random actions in victims, a Chaos Hammer spell-like ability, increased difficulty in hitting, and a permanent Protection from Law effect.
Xaositects are most known for (aside being a pain in the ass) their use of Scramblespeak, a bizarre cypher in which random words are jumbled out of order in various sentences at random.