The Druid Network
A British-based networking organization for Neo-Druids (including a North American members forum)

Neopagan.net
Current and archived writings of the founder of “Neopagan Druidism”

Mary Jones’s Homepage
Home to an outstanding collection of key Celtic texts and a “Celtic encyclopedia”

Internet Sacred Text Archive
As the name suggests, a collection of texts from sacred traditions around the world (mostly public domain translations, so often somewhat dated).  Includes a large Celtic archive.