These links are generally much more entertaining and instructive
than my usual links.
Gravitational Lensing
Joanne Cohn's excellent lensing webpage
Excellent review by Narayan & Bartelmann
Pete Newbury's Lensing tutorial
A Geometric Description of Lensing
Lensing a photo of a galaxy
Pete Kernan's "Lens an Astrophysicist!" Applet (here's an example of the result)
Fun Pictures
Astronomy Picture of the Day Archive
Hubble Space Telescope Images
Color Photographs from the Anglo-Australian Observatory
The Milky Way at 7 different wavelengths
An astronomy slide-show that I collected
Nice pictures of Gravitational Lenses
Entertaining Lectures and Simulations
Taking a Measure of the Universe, lecture by Harvard's Bob Kirshner
Physics 2000 -- cool physics simulations explained
Virtual Trips to Black Holes and Neutron Stars (MPEG movies)
Colliding Galaxies Movies
Andrew Hamilton's fun Relativity Simulations.
Instructive Notes
Ned Wright's Cosmology
and Relativity Tutorials.
Center for Particle Astrophysics: "Dark Matter" hypertext
Harvard's Astronomy 45 Class Notes
Astronomical Coordinate Systems
Some notes of mine on relativity and quantum mechanics
Martin White's Cosmology links
John Baez's more formal Relativity Tutorial.
Useful Resources
AstroWeb, links to everything
Sky & Telescope Magazine
National Radio Astronomy Observatory: Very Large Array
European Southern Observatory: Very Large Telescope (Optical)
PhysLink: Physics & Astronomy Reference
Chandra X-ray Observatory (formerly AXAF) Science Center