Powered by an incredible music discovery engine, Pandora has made a name for itself as one of the Web's most popular music services. There are versions available for several devices, and once you have the app installed, new music is just a button away. Pandora allows you to give it feedback, so if you like a song it'll play more similar tunes, and if you dislike it the service will remember not to play that song again. It's perfect for people who love music and want to find new artists without buying a ton of CDs.

If you use your mobile device as a music player as well as a phone and have free space on your SD card, you owe it to yourself to visit AmazonMP3. While it's not exclusively a mobile service, all of the music downloads are DRM-free, high-quality MP3s, which means they play on any device that plays MP3s, including your mobile phone. If your phone runs Android, you can download directly from your device.

Last.fm is incredibly popular for music discovery on your desktop or laptop using a Web browser, but the fun doesn't have to stop when you turn your computer off and pick up your phone. There are a number of third-party apps that let you take Last.fm with you when you head out, mostly for Nokia, HTC, and other devices running either Symbian or Windows Mobile.

Shazam is a relative newcomer to the music discovery club, but it's designed almost exclusively for mobile devices. Part of what makes Shazam so special is that you can use your phone to record a snippet of a song you're listening to and use Shazam to identify it and show you more by that artist. Tag music, explore artists rated highly by other Shazam users, and share your favorites on Facebook, all from your mobile.