I usually use them to blast out ideas and stick them to a whiteboard. I also use them to write down quick learning points, or draw flow chart on a white beforehand, then stick notes to the appropriate charts.
Obviously, people have more creative ideas than that. I came across a book called something like "1200 Ways of Using Post-its": make post-it happy birthday banners, piece them together into a big poster and even make a paper plant out of them.
Creativity never ends with it.