Lord knows why anyone would actually spend over $3 on a music video from iTunes when there are programs like this. Stereoclip synchronises whatever's playing through iTunes with videos of that track from YouTube. And it works great!
Powered by Adobe Air, Stereoclip is a free download for OSX or Windows. When the app is running, it will play a YouTube video clip of any song you are playing through iTunes, synchronising the time of the song to the time of the video.
When YouTube doesn't have an official video for that song - like album tracks or rare music selections, the app will find live versions or (if you're really desperate) videos of people singing/playing the song in their bedrooms. And if that isn't there, you just get an image of the album art, meaning you never have to completely go without something on your screen. It can all lead to sync issues being a few seconds out, but it's still a much better option than spending $3.39 a video.