Sierra is a video game company founded by Ken and Roberta Williams.
The company built its reputation as a company for excellent graphic adventures beginning in the early 1980s, and continued even after it was sold to CUC International in 1996. In December 1997, CUC International merged with HFS Incorporated to become the Cendant Corporation. After the Cendant book cooking scandal in 1998, Sierra was sold to Havas S.A, which was purchased by Vivendi (renamed Vivendi Universal in 2000 and now known as Vivendi SA) the same year. Sierra stopped producing adventure games in 1999. Vivendi merged with Activision (forming Activision-Blizzard) in 2008, and the entire Sierra staff was laid off and the Sierra building was closed. Sierra now only exists as a label.
The adventure games created with Sierra's Adventure Game Interpreter (and its predecessors) are implemented in ScummVM using the code base from the Sarien and TrollVM projects. The adventure games created with Sierra's Creative Interpreter are implemented in ScummVM using the code base from the FreeSCI project. All of them were done with permission obtained from the original authors of those projects.
There were several games that used some parts of the AGI engine, before it was mature enough (see TrollVM):
Starting in 1988, adventure game development switched to the more versatile SCI (Sierra's Creative Interpreter) engine. This new engine is object oriented, contrary to AGI that is procedural.
A version history of Sierra's games can be found under Sierra Game Versions.
Wikipedia article on Sierra