Gravitational Lensing

Gravitational Lens Data Base
Homepage of CASTLEs, a gravitational lensing survey based at the Harvard-Smithsonian Center for Astrophysics. The main feature is a data base, complete with images, of all gravitational lens candidates that astronomers have identified so far.
http://www.cfa.harvard.edu/castles/

Gravitational Lenses
Site hosted by the Université de Liège; features a bibliography, a list of lensing candidates, and descriptions of didactical experiments (involving glass lenses) useful for a better understanding of the phenomenon.
http://vela.astro.ulg.ac.be/themes/extragal/gravlens/bibdat/engl/grav_lens.html

Gravitational Lensing
A popular site with interactive demonstrations of gravitational lensing. The background material is at a graduate level
http://www.iam.ubc.ca/old_pages/newbury/lenses/research.html

Gravitational Lensing from a Spacetime Perspective
Article by Volker Perlick (Technical University, Berlin) published in Living Reviews in Relativity about gravitational lensing in curved spacetime (as opposed to descriptions that rely on slight relativistic deviations from the Newtonian-Euclidean description of gravity and space).
http://relativity.livingreviews.org/Articles/lrr-2004-9/index.html

HubbleSite News Center: Gravitational Lenses
List of images taken by the Hubble Space Telescope of Gravitational Lenses (including the famous Einstein Cross picture); updated continually.
http://hubblesite.org/newscenter/archive/releases/exotic/gravitational-lens/

Lectures on Gravitational Lensing
Lecture notes by Ramesh Narayan (Harvard-Smithsonian Center for Astrophysics) and Matthias Bartelmann (Max Planck Institute for Astronomy) about the basic concepts and applications of gravitational lensing.
http://arxiv.org/abs/astro-ph/9606001

Strong and Weak Lensing Constraints on Galaxy Mass Distribution
Review article by Jean-Paul Kneib about the way that gravitational lensing can be used to infer the mass distribution of galaxies (including the detection of dark matter).
http://arxiv.org/abs/astro-ph/0112123

Weak Gravitational Lensing
Article by Matthias Bartelmann and Peter Schneider about the statistical analysis of weak lensing effects that can be used to gain information about the large-scale mass distribution within our Universe.
http://arxiv.org/abs/astro-ph/9912508

Gravitational Lensing in Astronomy
Review article by Joachim Wambsganss, published in Living Reviews in Relativity. Provides a brief history of gravitational lensing, an overview of the basic physics and of the variety of lensing phenomena, and an outlook on future research. <small>(August 31, 2001)</small>
http://relativity.livingreviews.org/Articles/lrr-1998-12/