Chaosium (pronounced like chaos) was originally founded as The Chaosium in 1975 by Greg Stafford to publish the game RuneQuest (and the world of Glorantha). It was later incorporated as Chaosium Inc. It is based in the San Francisco Bay Area. (On an old army base, where they add that they are unable to dig for dinosaurs "by law.")
Chaosium is best known for their Call of Cthulhu game, which is the longest running horror-based roleplaying game on the market. In addition, Chaosium also publishes old HP Lovecraft novels to make sure that the old the fiction upon which Call of Cthulhu is based will stay in print. It is also well known for its work with RuneQuest (and the world of Glorantha) but that property is now owned by Greg Stafford and a separate company, Issaries Inc. In addition, Chaosium has released Nephilim, the Elric! roleplaying game, and the Pendragon roleplaying game. (They've won boatloads of Origins and GAMA awards for their work on these various games.) Since they've worked with Wizards of the Coast to produce a d20 System book for Call of Cthulhu as well as a d20 conversion of their Elric! game, I expect to see a bunch of d20 material coming from them as well.
They also published a CCG called Mythos based on CoC, which didn't sell entirely well, but was a tremendous amount of fun. They published some board games, as well, including Arkham Horror, which was one of the hardest things to find on the secondary market that I've ever attempted. (Easily exceeding $100 for a game that was originally about $20.) Since then, they've reprinted Arkham Horror, much to the dismay of my friend who bought it a month before for $75.
Currently, Chaosium's CEO is Charlie Krank. Lynn Willis is their line developer, Dustin Wright is their sales and support person, and David Mitchel is their Editorial Assistant. For everything else, they rely on freelance work. (Yes, they are a small company.) At one point, they also had Sandy Petersen as the line designer for Call of Cthulhu, and in fact, he had a lot to do with how well the game is presented now. (He now works doing video games, oddly enough.)
For more information, visit their website at http://www.chaosium.com/