It may be hard to think about the types of smart albums you can create, so use these ideas to get started.
Smart album ideas:
By matching on ratings, you can easily create a Favorites smart album that contains just your top-rated photos.
By looking for the filename extensions .avi, .mov, and .mpg at the end of an Any Text or Filename criterion (there's a bug with Filename such that it doesn't always find as many items as Any Text), you can create a smart album of movies.
Since iPhoto 6 adds Camera Model to the list of criteria for smart albums, it's easy to create a smart album that contains photos taken by cameras other than yoursin other words, photos that were taken by other people and sent to you. Of course, this doesn't work if one of your friends or family uses the same camera as you do.
To create an album that contains all the photos of a certain person, use the Any Text criterion matching that person's name. It won't be perfect, but if you give your photos descriptive titles or comments, it's a good start.
You can't remove photos from a smart album manually; the only way to take them out is to change either the photo or the criteria so they no longer match.