Slacker is available for free, but a subscription fee removes advertisements and adds unlimited song skips, lyrics, mobile caching (that lets you play stations offline, even without a connection), and an ABC News station.
1/11/2012 update: The following artists have custom-curated stations on Slacker:
The free Slacker Radio App gives you access to millions of songs from thousands of artists. Listen to over 120 expert-programmed radio stations or create your own. With a Slacker Radio Plus subscription, you can cache your favorite stations on [a variety of devices] and listen without a wireless connection. Unlimited free music..., over 120 expert-programmed stations, high-quality stereo sound over any connection, create your own custom stations, displays large album art, reviews and artist bios.
|iPhone & iPad||$0.00||$4.00|
|The $4/month option removes ads and adds unlimited song skips, station caching, news radio, and lyrics. The $10/month option has all of that plus on-demand playback of songs, albums, and artists, and the ability to create custom playlists. So basically, Slacker becomes an unlimited on-demand music service in addition to being a radio service. On top of either of those, a mobile $5/month brings it all to a variety of smartphones.|