Muve Music lets you search through thousands of artists and albums available on major U.S. record labels. You can download an unlimited number of songs onto your device for playback, and you can make ringtones as well.
The service also includes a DJ feature that lets you listen to ready-made playlists organized by your favorite music genres, Shazam integration, and a social networking arm that let's you "shout out" the current music you're digging to your friends on the Muve Music network.
Unfortunately, whatever music you download cannot be transferred to any other device, so it's pretty much limited to your phone. Also, once you stop paying your phone bill or cancel your plan with Cricket Wireless, you can't access any of your songs.
Right now, the no-contract, prepaid phone is on sale for $99.99 at Cricket Wireless. It's only being shipped to certain cities, so be sure to check if it is available where you are.