A thorough, beautifully illustrated tour of the eye's retina (with bonus material on color vision and on psychophysics).

Wilson Geisler's (University of Texas) website demonstrates the visual consequence of the human retina's uneven spatial resolution:

Delightful, intriguing illusions of lightness created by Edward Adelson and his colleagues at MIT.

A compelling animation showing a context effect in lightness perception (Barton L. Anderson, Jonathan Winawer (2005) Image segmentation and lightness perception. Nature, 434, 79 - 83).

Interactive demonstrations of the Hermann grid illusion. This site allows you to vary several parameters of the grid and see how the illusion is strengthened or diminished.

The concept of the receptive field is central to understanding vision and visual neurophysiology. For useful material on receptive fields go here.

Jody Culham (University of Western Ontario) has put together a very nice overview of brain areas involved in vision, including detailed images.

Matthew Schmolesky of the University of Utah vision group has created a richly illustrated website devoted to the primary visual cortex (V1), and it includes wonderful anatomical images and helpful diagrams.