An excellent site about Buddhist teachings in general, and Buddhist meditation in particular. The site is also a great resource for writings by the Venerable Mahasi Sayadaw and other authors. Contains links to many books, dhamma teachings, dhamma talks, and a discussion forum too.

Vipassana Fellowship's online Meditation Course provides a supported introduction to Buddhist Meditation as found in the Theravada tradition.

This is the international home page of the organizations which offer courses in Vipassana Meditation as taught by S.N. Goenka and his assistant teachers in the tradition of Sayagyi U Ba Khin.

Wildmind provides a complete guide to simple meditation techniques.