HTC has released a number of phones sporting "Beats Audio", a branded equalizer that boosts the bass, among other things. XDA modders RockoDev has turned this into a flashable ZIP file that works on any phone running Gingerbread or a Gingerbread-based ROM.
While the best way to improve your music would be to use an advanced equalizer like that in PowerAMP, our favorite music player for Android, those that don't really want to go through the work of setting up an equalizer 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). The above images to the right show the difference before and after the mod has been applied on a Samsung Galaxy SL—the first image uses default audio, while the second image shows sound with the Beats mod installed. 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.
Update: The thread below has been removed. You can still find the necessary files here, but you'll have to place them into the correct destinations with something like Root Explorer instead of flashing it.
Beats Audio for All Gingerbread Roms | XDA Developers via XDA Developers Blog