If you regularly use iTunes and LastFM then you're going to love SweetFM. SweetFM allows you to combine the best of both applications under one controller. It used to go for almost $30 but right now it's completely free and open source.
SweetFM switches seamlessly between Last.fm and iTunes. If you get bored of listening to either application at any point, you can just switch in a click. You can configure hotkeys to control most tasks and even use your Apple Remote - useful for parties or just kicking back and relaxing.
But there's more to it than that. SweetFM extends Last.fm functionality with things such as tag-station shuffle. Just enter an artist name with some keywords and let Last.fm create the playlist for you. You can see what's next on your playlist in the dock or the status bar menu. I like the fact that SweetFM also uses album art covers retrieved from Amazon.com when playing major artists. When you first start it, this can take some time to appear though and retrieval in general tends to be a little slow.
One other thing worth bearing in mind about SweetFM is that if you want to export tracks to iTunes, it will automatically check for duplicates although this obviously relies on your music being tagged correctly. There's lots more to SweetFM than this too including CSS skin design support, Skype moods integration and handy Growl notifications.
There's little to fault with SweetFM and considering it's now completely open source and free, the time has never been better to try it.