We've been covering a lot (a lot!) of iPhone games in this space, but of course there are many other types of apps on the App Store. MusicPound isn't a game -- rather, it's a music discovery app that's designed to compare your music library (on your iPhone, so you'll have to load music on there if you don't sync it usually) with someone else's. The app will give you a "Music Compatibility Rating," as well as chart your most played and favorite songs, and give you some insight on what kinds of music you both listen to.
Obviously, the feedback here is mostly just for fun -- you're hopefully not going to keep or ditch a friend based on what this app tells you. But the app can give you some good recommendations based on what kind of music your friends like, and where your libraries intersect and diverge. And of course MusicPound lets you jump into iTunes to fill in any gaps in your library.
And the app itself is free, so the price is right. You get three comparisons with your free download, and if you want more, you can do an in-app purchase for just $1.99. MusicPound probably isn't the only music recommendation app you'll use (I really like Pandora and Shazam as well, both apps that I've mentioned before), but with a willing friend in tow, it can add a fun social twist to finding new music.