Rdio — Listen to any song for free
More and more people are swearing by Rdio. It's an online music service that allows you to listen to any song you want, anytime you want, anywhere you want, for a flat monthly fee. It's like an all-you-can-eat iTunes in the Cloud. There's a beautiful website, and iPhone and iPad apps to go along with it. Plans start at $5 for web-only and $10 to use the apps as well (although keep in mind your data-cap as you'll be streaming those songs). But you probably knew all of that already.
There is also, however, a totally free Rdio plan. Just sign-up for Rdio and you'll be, by default, setup with the free plan. So if you've never really looked at Rdio because you thought there was some sort of cost associated with it, it turns out that there isn't. You only get a limited amount of music per month (but it's quite a lot), and there's a little green meter at the top that shows you how much you have left. And it's web-only. Other than that though, you get everything else. Any song, whenever you want. Totally free. That's a great thing to have access to at your desk. There's also the neat social integration so you can see what your friends (who's musical tastes you may or may not share) are listening to on "Heavy Rotation" (as Rdio calls it). Rdio used to also have a fantastic, supposedly, recommendation engine but that disappeared with the latest update. Rdio promises that they are working to reincorporate that, though.
What are you waiting for? Go sign up.