Chaosium is one of the longer lived publishers of role-playing games still in existence. Originally founded by Greg Stafford, its first game was actually a wargame, White Bear, Red Moon, which later mutated into Dragon Pass and Nomad Gods. White Bear, Red Moon is notable as containing the first published material about Glorantha, later immortalized as the primary setting for the role-playing game RuneQuest and, now, Hero Wars.
The rules of RuneQuest, Chaosium's first role-playing game, were distilled down into a generic, genre-agnostic format known as Basic Role-Playing (BRP). These generic rules formed the basis of many, if not most, of Chaosium's later RPGs, such as Call of Cthulhu, Stormbringer, Nephilim, and Ringworld.
Chaosium and Greg Stafford are also responsible for Pendragon, an Arthurian RPG now published by Green Knight Publishing. Other games of note include Mythos, Elfquest, Worlds of Wonder, Superworld, Hawkmoon, and the frequently forgotten Prince Valiant.
Some Chaosium products have been translated into French and are available in France from Multisim.
In the mid 1980s, Chaosium entered into a complex arrangement with Avalon Hill to publish RuneQuest material while Chaosium maintained editorial control over Glorantha-based material for the game (which Avalon Hill would publish). While this agreement reportedly kept the company in existence, it also effectively killed the game of RuneQuest, leading to products of questionable quality, long gaps with no products published at all, and, eventually, the death of the game altogether.
In the late 1990s, Chaosium sort of fissioned into several separate companies. Green Knight Publishing formed to focus on Pendragon, Chaosium proper retained Call of Cthulhu, Stormbringer, and Mythos, while Issaries formed to publish Hero Wars and focus on bringing new Glorantha related material into print. Also, Wizard's Attic was formed in order to act as a fulfillment house.
Chaosium's official website can be found at http://www.chaosium.com.