Might I suggest- http://www.slacker.com/
I've been using it since it came on-line earlier this year and love it. NO COMMERCIALS, JUST MUSIC 24/7.
Although navigating the user-interface takes some getting used to, it not only allows me to create free "stations" and pick my favorite artists, based on my ratings, I can limit it to just my favorite song titles. Other people can listen to my stations, (not that I want the world to know I only listen to old show tunes and live metallica concerts) but only I can decide what the content is. As Slacker expands their artist inventory, they claim they won't even have to fill the occasional gap with "similar artists". They've already got most of the artists I like, so that hasn't been a nuisance.
Navigating starts by hunting for the tiny sign in hyperlink near the upper right of the main page.
Go through the free registration process (no spamming that I've seen) and then follow instructions to create your own station and pick your artists. Note: You have to pick a certain number of artists (10 or 15?) for it to be treated as an active station)
To the original free on-line webplayer service (which I love), they've added a Premium radio service, a software player, and now they're touting a portable version -
http://www.slacker.com/products/portables/
FAQs are buried. You have to click on one of the service icons here -
http://www.slacker.com/support/
Despite its non-intuitive layout, I'd rate Slacker 4.5 RadioTowers.