It's new. It's updated. It's actually pretty boring. iTunes 5 offers the all the same things you've come to expect from the program, with a slightly updated interface, a couple of new features like the playlist folders and some additional icons (Podcasts and Party Shuffle).
I'm not saying that iTunes isn't an excellent program, just that there's not much new or interesting in the latest release. On the bright side, you can click on a playlist folder and see all the songs in all the playlists in that folder, which is very useful indeed, and the search bar has been updated nicely. Free download, Windows or Mac.
iTunes 5 [Apple]