Edit: On closer inspection, we are much less impressed with the resolution of the Clique camera. The Markvision camera displays a much clearer image, but is no longer available. Stay tuned for some pictures of webcams/cameras mounted on stands allowing them to function as document cameras, similar to what JWatson14 mentions in the comments.
If you've ever looked longingly at document cameras but realized that there is no money in your budget for one, you could perhaps use a cheaper webcam instead. There are even webcams with stands and flexible necks that allow them to be used like a document camera.
The Clique CAM00131 Hue HD Webcam is an example of one of these, and it's currently on sale for $39.99. We've done some testing with it, and are very impressed. It has great framerate (30 frames per second), good resolution (1.3 megapixels), a long enough USB cord (although you could use a USB extension if needed), and feels fairly sturdy. The camera can either be connected to the the base or to a USB port on the side of your laptop or monitor. There's a button on the top for taking snapshots, and a built-in microphone. It comes with fairly good software, allowing things like image flip and mirror (recommended) and digital zoom. The camera and built-in microphone also work with programs such as Skype and Ustream.
A few minor issues are that it has manual focus, only digital zoom, and the neck may be a little short for doing dissections underneath the camera. You could always just put a couple of textbooks under the base to raise it up.
There's a similar webcam available, the Markvision MVWC-SN4923 Snake Webcam. It's only $23.99, but it doesn't feel quite as sturdy, has a shorter neck, and some computers we tried had issues with the driver. TigerDirect also tells me that they have limited stock of this one, and don't know if they'll be getting more.
TigerDirect.ca is currently offering free shipping for orders over $100 and under 20 lbs. Our purchases arrived a week after ordering.