You can make your own USB microscope using an old webcam and a cheap toy microscope. All it takes is a soldering iron, some hot glue, and an hour of your time. Instructables user moris_zen has the details, which basically involves a little focusing trial and error before gluing things together:

While you view the image via the SW used for the webcam – position it so you get a clear image. You may need to play with it for a while. After you find the exact location use a hot glue gun to fix it to that position. Then tape the wires tidily to the microscope and start taking pictures…

I imagine you’d get even more interesting results substituting the old 320×240 web cam for an old 2 megapixel digital camera.

Also worth checking out would be to skip the optics entirely, positioning the CCD right up against the sample with a proper light source. I believe this is the technique being used in the UCLA cellphone microscope. The output is supposedly low-res and blurry, but it’s decent enough to capture the shadows of individual cells, making cellcount-based diagnoses possible.

Build a USB Digital Microscope
Aydogan Ozcan, UCLA Lensless Imaging System