iOS: Aphex Xciter is near-identical your iPhone's standard music app, but it processes the signal to add clarity and richness to your music and other audio content.
I tested the app on various types of music, and I found it dramatically improved more complex pieces, but didn't make much difference with simpler recordings. Essentially, it works better on an orchestral piece than a piano solo.
One downside is that it won't play copy-protected tracks, which could be a problem with older tracks from iTunes. While there's a free trial version, it only lets you listen for a short amount of time each day. For unlimited listening, you'll need to pay $2.99 for the full version.
Aphex Xciter (Free Trial, $2.99 for Unlimited Play, $5.49 for Studio) [iTunes App Store]