All Beliefs are Welcome Here!
This is a link roundup of text sources. Some are large collections; some are single books. Some are articles and not easily downloadable.
Sacred-texts.org - this has been around basically forever, and was one of the earliest spiritual text archives online. Their Book of Shadows section is not the same as their Wicca and Neo-Paganism section, although I believe there is some overlap. These are the texts a lot of us who got our start in the 80's are familiar with - they were often handed around from photocopied versions in binders.
Project Gutenberg is not specifically a Pagan site; it's for public-domain ebooks, and it's been converting texts and sharing them online since the mid-70's. They have a category for Paganism. It includes literary classics like The Children of Odin: The Book of Northern Myths by Padraic Colum (first published in 1920), Legends of the Gods by Sir E. A. Wallis Budge (Egyptian lore, 1912), The Fairy-Faith in Celtic Countries by W. Y. Evans-Wentz (Celtic, 1911; still used by many Faery witches today), and The Witch-cult in Western Europe: A Study in Anthropology by Margaret Alice Murray, which has some serious gaps in its fact-finding, but is part of the history of modern Paganism. It also has fiction and mythology - the Fairy Books by Andrew Lang has all 12 of the color-fairy books (red fairy book, green fairy book, etc.) compiled into one ebook.
Mike Nichol's Witches' Sabbats website has good articles on the 8 sabbats.
Barnes & Noble has a small collection of free Pagan ebooks.
Archdruid Report on the wayback machine - John Michael Greer's weekly posts for over 10 years, discussing the limits of planetary resources and related politics. There's also a mirror site with perhaps better formatting, but without the archives of the discussions in the comments. (He gave plenty of time for people to make mirrors and backups there are several.)
Witchcraft Training (wayback links)
The Dragon Wheel Teaching Tradition coursework is online, although the forum it's attached to is dormant.
Compost Coven Disc of Shadows including rituals of early Reclaiming and some resources of the Feri tradition
Witchcraft From the Ground Up - from the old WhyWiccansSuck.com website, one of the most comprehensive introductions to non-religious witchcraft on the web, and at the time it was live, possibly the most complete.