Rdio is aiming for Spotify with a free plan that'll provide full access to Rdio's entire music catalog. There's no advertising and no need to add a credit card to your account. The free service will launch very soon. [Rdio]
I love Rdio.
I tried all three major players, Rdio, MOG, and Spotify, and Rdio wins hands down.
Rdio may not provide the best sound quality of the 3, but it provides the best user experience on any platform. Not only does it have an online web app (Spotify fails so hard for not having this) but its mobile apps are far superior to the others.
The only problem with Rdio is that most of my friends are on Spotify since its big in Europe and had more marketing recently here in the states.