Slacker is a service that, like Pandora, lets you identify the type of music you like and then creates a personalized station that streams this music. And Slacker, like Pandora, makes it super easy for you to create your station, beginning simply by naming your favorite song or artist. These two websites are hugely popular, and both offer a free iPhone app that lets you access your stations or create new ones.
Slacker’s music catalog has over 2 million tracks. Slacker Pro ($3.99 per month) lets you have unlimited skips, unlimited song requests, and lyrics to the songs — as well as being ad-free. You'll want the pro version because, unlike Pandora, Slacker occasionally has commercial interruptions.
What I find is that I like using both Pandora and Slacker. My favorite station on each is the light classical station I've created starting with the colorfully orchestrated Russian Easter Overture by Rimsky-Korsakov. Both Pandora and Slacker stream orchestral selections in the same vein that are delightful, but they're different enough that I like having both.