Dwarf Fortress is an odd game. It looks as if it was made in 1990 but it will eat up all the CPU cycles of your modern, high-end machine, I guarantee . It's incredibly complex, because the developers only worked on the gameplay elements instead on focusing on the graphics like most other game. The result is an overly complicated game which will take a while to understand. But if you do, I guarantee that it's gonna be a lot of fun!
If you like games that are a bit different, you should definitely give DF a try.