Olympus Microscopy Academy
Carl Zeiss Microimaging Online Campus
Nikon Microscopy U resource centre
Molecular Expressions microscopy primer We are going where no microscope has gone before by offering one of the Web's largest collections of color photographs taken through an optical microscope (commonly referred to as "photo-micro-graphs"). Visit our Photo Gallery for an introductory selection of images covering just about everything from beer and ice cream to integrated circuits and ceramic superconductors. These photographs are available for licensing to commercial, private, and non-profit institutions.
Wikipedia's excellent microscopy overview
The Royal Microscopical Society