There are many sites offering free clipart (usually as vector graphics). Many have annoying pop-ups and banners, many are way too specific to be useful to any one but that one or two guys out there. But if I’m looking for something particular, I have found the Open Clip Art Library to be a very valuable resource. It is not very “browse friendly”, so you can go and just surf around a bit. You’ll just encounter a lot of junk. If you have something specific in mind, just search and you’ll be surprised how vast, detailed and qualitative the collection is.
I used Gerald G’s great work to spice up Recipe Book 2.0 a little. Yay for burgers.