In Chronological order.
Eric P. Lafortune, Yves D. Willems, Compugraphics '93
Introduced bi-directional path tracing.
Eric Veach, Leonidas J. Guibas, SIGGRAPH 95 Proceedings (August 1995)
Paper on multiple importance sampling.
Eric Veach, Leonidas J. Guibas, SIGGRAPH '97 Proceedings
Introduced metropolis light transport.
A. J. Preetham, Peter Shirley, Brian Smits, SIGGRAPH '99 Proceedings
A nice physically based spectral sky model that i'm using.
Practical and Robust Stenciled Shadow Volumes for Hardware-Accelerated Rendering
"We describe a robust, artifact-free technique for hardware-accelerated rendering of stenciled shadow volumes. Assuming existing hardware, we resolve the issues otherwise caused by shadow volume near and far plane clipping through a combination of (1) placing the conventional far clip plane .at infinity., (2) rasterization with infinite shadow volume polygons via homogeneous coordinates, and (3) adopting a zfail stencil-testing scheme"
Paul Debevec's home page
Lots of cool stuff from the master of HDR image based lighting.
The Cornell Box
Some info on the notorious 'Cornell box'
Monte Carlo Ray Tracing - Siggraph 2003 Course
Lots of tutorial type papers on Monte Carlo Ray Tracing. Very hardcore maths :P
Poynton's Gamma FAQ
Stuff about gamma you didn't know you didn't know :)
TBP's site which has a forum where you can discuss kd-tree building algorithms 24 hours a day if you're so inclined.
Igneus GI renderer
Interesting Dev blog about a photon-mapping ray tracer with some cool features.
OpenGL Extension Registry
All the OpenGL extension txt files.
New Zealand Game Developers Association
Penny Arcade: Funny.