GeneRally is a fun top-down racing game with loads of customization in it. If you're good at it, you can make your own tracks, cars, sounds for the game... loads of stuff! You can download it for free on gene-rally.com.
Wouter (on the GeneRally forums): I made a new track!
Everyone: *doesn't comment even though Wouter's track is a masterpiece*
(yes, this is a crap example but this is something that happened to me this morning)