I'm going to be tooting my own horn, so I apologize, but a really great application for the iPhone is DOT.TUNES.

DOT.TUNES is a cross platform web-based application that lets you access your iTunes library remotely across any device with an internet connection. The application, which is free, is available at http://dottunes.net, and is customizable with a suite of plug-ins (which you've got to purchase), to enhance your experience.

The iPhone plug-in has a customized interface which allows you to access all of your movies, videos, podcasts and music directly from your iPhone. Once you download the application, and purchase the plug-in, you'll immediately be able to access your entire library from your fingertips.

You can stream your music, watch movies, create playlists, rate your music, see album artwork, all from the iPhone interface, which is clean, intuitive and simple to use.

One downside is that you can't stream one song after the next, but I'm certain that subsequent versions will work that out. Its also best when experienced over wifi, since the Edge network is notoriously slow. However, even over Edge, streaming mp3s is still quite pleasing.

I said that I'm tooting my own horn because I'm the Product Manager for DOT.TUNES, so excuse my lack of impartiality, but it is honestly one of the best applications available for the iPhone today (and I think its the only one) that lets you access and stream remotely from your iTunes library.