Multiplayer browser game
http://en.wikipedia.org/wiki/List_of_multiplayer_browser_games
List of freeware games
http://en.wikipedia.org/wiki/Freeware_games
List of open source games
http://en.wikipedia.org/wiki/List_of_open_source_games
Open Source Games for Windows
http://osswin.sourceforge.net/games.html
Game For Linux
http://games.linux.sk/
http://www.happypenguin.org/
Linux Ubuntu Games
http://www.ubuntugames.org/
http://www.playubuntu.com/
https://help.ubuntu.com/community/Games
http://ubuntuforums.org/showthread.php?t=427205