Seeqpod, the music search service just released a version of their service for the iPhone. The service allows you to browse and search for mp3s accross the Internet and stream them directly to your iPhone. You can browse popular artists alphabetically or use the search feature to locate an artist of your choice. They have also included a discover feature that allows you to find songs for related artists. Once you locate a song you wish to hear, just tap it with your finger and the mp3 plays back using Quicktime.
The browser version of Seeqpod allows you to add found songs to a playlist and view YouTube videos but these features are currently unavailable in the iPhone version. Like the browser version, the iPhone version does not allow you to download songs directly.