Remove broken links and duplicate songs from iTunes

Formerly known as meta-iPod, meta-Tunes will let you remove duplicate tracks and also lost links in your iTunes library. While you are at the Macroplant you might want to have a look at Adapter, a very easy to use video converter for your apple devices with pre-configured settings for iPod, iPad, iPhone, Apple TV and iPod Nano among other.
They also have Sharepod (though not freeware) which will let you copy music from your ipod or iPhone to your iTunes on your PC or Mac