FreeCiv is a free multiplayer strategy game, comparable with Civilization. It runs on Linux, Win32, etc.
Play some of the classic Infocom text adventures online (titles include the Zork series, Suspended, etc.). These adventures can also be downloaded and played on your PC using Windows Frotz. Another site for adventure games is the Interactive Fiction Archive. The Infocom Gallery has the maps, inserts, etc., that were originally shipped with the games available for viewing.