Mediamonkey is good for this, as others have mentioned, although I had some problems with embedded album art using it.

I use mp3tag now, and get the album art via albumartdownloader, which searches multiple online cover art repositories. I find this combo best since I albumartdownloader lets me choose nice clear images, usually at 600x600 pixels (which is increasingly important as mobile displays keep improving), and because mp3tag just works with everything I've thrown at it - my PS3, WinMo phones (in all three media players on my last WinMo phone too), Symbian, Windows... Its good to have a solution that works when you change OS environs so often...