Over the years, I've accumulated a lot of photos in my iPhoto library. Usually I import photos and move the ones I like to an album. It didn't occur to me that all those left over (bad) photos were still taking up space in the Library.
Instead of comparing the Library to each album by hand, I created a Smart Folder with the following condition: Album is not Any. As a result, all images in the Smart Folder were images that were not in any album, and therefore, I could safely trash them (all 300!).
Mac OS X Hints
http://hints.macworld.com/article.php?story=20050508180050685