This site by Vere Chappell offers a great introduction into Thelema and covers all the basics a newcomer would want to know on Crowley, Liber Al, the Stele of Revealing, Magick, thelemic calendar and some of the main organisations in order to get a rough idea on what it's all about. It launched in the very early hours of the WWW and was back then as popular as it still is, I hope.