Pete's Power Point Site is one that I happened upon this last week. This site has a multitude of power point presentations to use or look at for free. The presentations themselves are in various categories - art, history, math, technology, and more. These slide shows go nicely as supplemental information with whatever you happen to be teaching.
For our own use, we wanted to look at Byzantine art as a part of our history lesson. It was tedious to try and find different art to bookmark and then come back to for our lesson. I just happened upon Pete's Power Point Site and found a slide show that fit our needs very nicely. I can also see how some of these might come in handy for a co-op or group situation.