Gnome runs, but you have to install XFree86 and fink beforehand. Both are free from Apple's site.
Games (at least most of the ones I've used on it) scream, provided that they're Carbonized.
Well, it's based off of BSD, so it's very different from linux, however the command-set is very, very similar. Takes a bit of getting used to, but hey, most Unixes do.
You can recompile and run most opensourced unix software under Mac OS X, either using fink or the cc compiler. However, if you wish to purchase software, there's a number of Carbonized programs available for almost anything you'd want.