Sierra On-Line, later known as Sierra Entertainment, was founded as On-Line Systems in 1979, by Ken and Roberta Williams. They became known as a premier publisher of graphical adventure games, particularly their "Quest" series of games, such as King's Quest (designed by Roberta Williams), Space Quest, and Police Quest, as well as Leisure Suit Larry and other games. Later their lineup diversified with action games (they were publishers for Value's Half-Life), puzzle games (the Dr Brain series) and even an early dial-up online service (The ImagiNation Network). Today the brand (after being bought and sold) has been "reimagined" and continues to produce games. See also www.sierragamers.com for the website of original founders Ken & Roberta Williams.
Official Sierra On-Line website
Sierra On-Line's Mobygames page
Sierra On-Line's Wikipedia page