POVRay
The Persistence of Vision Raytracer

POVRay Links
Collection of Links on the Official POVRay Site

MegaPOV
An Offshoot of POVRay with some very nice features

Oyonale
A fantastic site with some amazing images and sources, along with tutorials, including an excellent one on Clouds

Persistence of Ignorance
Very Nice Site with Rendering Tests, Sources, Tutorials, and plenty of Links to other POVRay Sites

Poser to POV
Mike Williams's Poser 3 models to POVRay 3.5 Tutorial

Isosurface Tutorial
Mike Williams's Fantastic Introduction to IsoSurfaces.

The Raytracing Repository
An Introduction to Raytracing, as well as instructions to write your own

Christoph's Page
Splendid Site with Comprehensive Tutorials, and quite good include files and tools (Home of IsoCSG and IsoWood)

GrassPatch
An Include File and Tutorial for Making Realistic Grass

WikiPov
The POVRay Wiki. Yep.