Spelunky is a fun little platformer, exploration type game. It has some elements of roguelikes like the fact that there are no saves and the maps are randomly generated. The goal of the game is to collect treasure and to get to the ending.
This game is loads of fun for a lot of reasons. Although it is very hard, and you will die a lot at the beginning, it's fun to play. The best thing about it is discovering all the different things you can do in game (like trying to kill the shopkeeper, or sacrificing the girl at the ritual table), and it makes for some great gameplay. And even though there's no saves, every 4 levels or so there's a shortcut digger who, if you pay him enough money (will probably require visiting him a couple of times), he will dig a shortcut to that level and then you can start from that level anytime.
Here's the link if you want to try it:
Spelunky World (Windows only)
