Open Source and the Gaming Industry
Many companies are taking advantage of open source software, much in part to the increasing popularity of Linux. Open source has become a big buzzword, with many people not really understanding what it really is.
In fact, most people are using open source software and don't even realize it. When you are looking at a website or checking your email, chances are the inner-workings (the mail transports and web servers) are open source.
Open Source Games
There are several open source games available. Some games have been released as open source, some have become open after a commercial release. On Wikipedia is a list of open source games, I'll highlight a few here.
Parallel Realities has a number of open source games including Blob Wars: Metal Blob Solid a 2-D arcade game. A cool thing that Parallel Realties does is release the making-of for several games.