If, like me, you live Down Under and don't have access to some US-based music services like Pandora or even Amazon music downloads, then you might like LoadIt.
LoadIt is a music download service which sells tracks at $1.69 each (or, for $29 a month, you can download 300 songs a month). I haven't bought any tracks yet - but I'm loving the 14-day trial where you can download up to 300 free songs (that stop playing after the trial period).
LoadIt uses evil Digital Rights Management ("DRM") which basically means when you buy the songs, you can only play them on a couple of PC's using Windows Media Player, and it works with Windows only as far as I can tell. It's advantage is that it's tightly integrated with Windows Media Player: songs you already own are marked "already in library", you can search using the familiar WMP interface, and the catalog of over 1 million songs is browsable by the recognisable album, artist, year, genre and more:
I hope this service gradually loses DRM and increases the size of the library. At $29 a month - less than 2 full-price CDs - it might be worth it.