ArenaNet is a game developer that initially consisted of many prominent employees of Blizzard Entertainment. Their chief focus is the successful Guild Wars franchise, which is an MMORPG that is free to play.
ArenaNet was founded by three influential ex-employees of Blizzard.
Mike O'Brien was one of them, and he was responsible for the rendering engine for WarCraft III, and also created Blizzard's mpq archival format which is still used (albeit modified) today. He also led the development of Battle.net.
Patrick Wyatt was formerly one of the lead programmers for Blizzard, and had a large part to play in the construction and continuation of Battle.net, and the multiplayer components for Blizzard's two RTS series.
Jeff Strain is the third founder of ArenaNet and also served as a programmer on the World of WarCraft team, in addition to creating the campaign tools for StarCraft.