Ever since my Klipsch 2.1 speakers died I've been using a pair of M-Audio monitors for computer speakers. While they are good speakers, especially for voice (i.e. movies), they lack severely in the low end department being a 2.0 system.
I listen to music quite a bit while sitting at the computer - more than I should - so I finally decided to get a proper 2.1 system for my new PC. I ended up with the Logitech Z-2300 system that seemed to be getting some great reviews for the price. So far so good with them and I'm still using the on-board sound. The sub seems to be pretty damn good for the size (no farting out yet). It's nice to finally get back those sub 100Hz frequencies that the M-Audio speakers couldn't reproduce.