BioInteractive is a site from the Howard Hughes Medical Institute (HHMI) that provides free resources for science teachers and students.
The site has videos, lectures, animations, links, downloads, lesson ideas, virtual labs and much more. You can even order DVD's of many of their programs for free.
The virtual labs include: Transgenic Fly, Cardiology, Bacteria, Neurophysiology, and Immunology.
The materials were developed by educators and scientists and are excellent.
The HHMI is a great resource itself.