Webvision. Comprehensive source of information regarding the visual system.
The Brain from Top to Bottom: The Retina. A comprehensive guide to the brain by McGill University. This page has to do with the organization of the retina, but there are many other pages on this site related to the visual system.
Receptive Fields in the Retina. A flash-based animation tutorial about the receptive fields of retinal ganglion cells.
Colour Aftereffects: Colouring a Black-and-White Photo with your Mind. A very interesting demonstration of color aftereffects.
Colour Aftereffects: A full colour ad that moves with your eyes. A compelling demonstration of color aftereffects.
The Thatcher Illusion. A visual illusion that indicates something about how the brain processes information about faces.
Jane Goodall and Prosopagnosia. Some evidence from Jane Goodall was a prosopagnosic (and that Oliver Sacks might have been one too).
Motion aftereffects. Motion perception produces aftereffects, much as color perception does.