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Windows stress relief, although it doesn't relieve stress, it is kind o' fun to beat the living daylights out of your desktop with machine-guns and lasers.
Trilby: The art of theft: a fun game where you rob buildings and hotel suites
6 days a sacrifice: another trilby game
Trilby's Notes: another Trilby game
7 days a skeptic: yet another brilliant Trilby game
5 days a stranger: another awesome Trilby game
The Mana World - A free 2D MMORPG, similar but better than, runescape
Steam - A free gaming platform by valve software to help make accessing games easier, get many free demos and downloads
Starport GE - A free MMORPG set in space, very good