This game is great, it is so simple, yet so addictive and fun.
It is challenging and quite unforgiving, as you have to start over when you die. Yet it has great replayability as no playthrough will ever be the same.
And it's always fairly cheap :)

The ultimate sandbox game! Can be great fun to build massive wrecking machines or to pose the characters in weird positions, and the modding community is massive! (www.garrysmod.org)

A great game, its a mix of FPS and RPG and will challenge your skills constantly. One of the interresting features, is the infinite weapon diversity. The game uses weapon templates and adds random bonusses and disadvantages on every individual weapon, like in the Diablo games.