I really like Apple's Remote application on the iPhone, which lets you use the phone like a remote control for iTunes, complete with library browsing and display of cover art. However, in the living room, I tend to listen to my music collection using my SliMP3 player, via SlimServer running on the iMac upstairs. So I was delighted to find a plugin for SlimServer, called iPeng which does the same job as Remote for my SliMP3.
It's not actually a native iPhone application: you access it via MobileSafari using a special URL, and it shows you a very nice interface for your library, with full control over playlists and so forth. However, it's easy to put an icon for the URL on the main screen, so in practice, you hardly notice that it's not a native app.
I do have a standard remote control for the SliMP3, but it's much easier searching and browsing my music collection from the iPhone, rather than fiddling about with the limited buttons on the physical remote. I love the way I keep finding additional uses for my iPhone -- it's certainly the most versatile gadget I own. Somehow, it manages to be a Jack and Master of All Trades.