Virtual microscope #1
This takes you to a place where you can control many scanning electron microscope images - it takes 2 minutes to load, but it worth the wait. (You may never return here!)

Virtual microscope #2
This does exactly what is says on the tin - click on the eyepiece to view a large range of excellent microscopic images

Virtual microscope #3
This links to a short piece of interactice Flash animation from www.cellsalive.com which might help you understand the relative sizes of some microscopic creatures. It may take a few minutes to open, but it will also be worth the wait!