I had this idea when I was at a lyrics site trying to print out some lyrics. The page didn't have a good print stylesheet and was full of all kinds of crap. I copied the source code and made a local version of the site and dumped some simple jQuery stuff in there to make it so any thing I clicked on was eliminated from the page. Turns out it's the current version of the DOM that gets sent to the printer, so I could eliminate things at will and have them not print. Cool!
After the idea was in place, I went back and forth with Devon Govett turning it into a more full-featured bookmarklet. The current iteration has click-to-remove elements, with a really cool feature of option-clicking to remove everything except the selected element. There are more features, and I think the video explains it better than I can with words:
To grab it, head over to the demo page and drag the bookmarklet link into your bookmarks bar. Click it on any page to activate.