I usually think of online radio service Slacker as a direct competitor to Pandora: both offer user-customizable radio stations based around individual tastes, both are available for the iPhone and other mobile platforms, and most important, both are available for free, with optional subscription versions--Slacker Radio Plus and Pandora One--offering additional benefits.
But unlike Pandora, Slacker also has a big investment in more traditional one-to-many Internet radio, with actual humans programming stations or playlists based on particular genres. This is nice if you really don't know what you want, or simply want to be surprised. This week, the company took this programmed radio model another step and introduced stations curated by alternative rock band AFI and hip-hop magazine The Source. In each case, the curators not only pick songs, but record introductory spots or between-song banter. Slacker automatically inserts this material between songs the first time you play a particular track on that radio station, then makes sure you don't hear that same banter again. It's sort of like a podcast, but constantly changing, and with no fixed length.
If neither of these stations appeal to you, stay tuned: Slacker expects to add more stations curated by artists and music publications in the next few months.