Jonathan has put together a collection of downloads that contain clipart for use in PowerPoint (or whatever your favourite non-Microsoft presentation tool is).
I have to admit to having a slightly less polished version of similar clipart that I use myself – it’s good to see that I’m not alone! I’ll try and make some time to tidy mine up for posting here, but in the meantime, if you need some good clipart, I can strongly recommend that you take a look at Jonathan’s collection.