I'll second DFX. It makes an incredible difference in how your mp3s sound. Download it from Winamp and pay to register it. It is SO worth it.

The crossfeed speaker simulator, as it's the only one I've tried. Now if winamp would stop the burp of sound everytime I start a new track (not related to the crossfeed plugin as far as I know).

Google Operating System has a few tips to make Winamp, a freeware media player, sound even better - and no, there's no need to buy a new sound card. Instead, you can check out the MAD MP3 decoder, ...