HTC has released a number of phones sporting "Beats Audio", a branded equaliser that boosts the bass, among other things. XDA modder fuss132 has turned this into a flashable ZIP file that works on any phone running a Gingerbread-based ROM.
While the best way to improve your music would be to use an advanced equaliser like that in PowerAMP, our favourite music player for Android, those that don't really want to go through the work can flash the Beats Audio mod to their device instead. Just download the ZIP file and flash it like you would any other ROM with ROM Manager, ClockworkMod, or your recovery of choice.
Some people are finding their music sounds better with the mod, others are having trouble seeing a difference (though you'll probably notice it more with a nice pair of headphones). Hit the link to read more and to download the mod.
And please, guys, let's not get into a flamewar about Beats in the comments -- some people love it, and that's fine for them. Let's keep it friendly.