If you haven't heard of StumbleUpon you are probably still a very productive person. If you want to make your time wasting sessions slightly more efficient, you can use the keyboard to stumble instead of clicking the Stumble! button.
I've calculated that you could save an entire 32 seconds per year this way, which is at least one more stumble (well worth it). It will also allow you to stumble without having the toolbar visible, so you have more screen space to stumble with.