The Vault of Origins
Another great site dedicated to the Sisters of Battle

The Games Workshop Gateway
The first portal dedicated to Warhammer

The Games Workshop Message Board
Online message board for veterans and newbies alike

Portent.net
A great site for Warhammer rumors and other goodies

The Tomes of Knowledge
Loads of background stories about the Warhammer Universe

Dakka-dakka.com
A 40k League site with lots of action as well as information

The Colossal 40k Resource Site
A great place for 40k-related information and utilities

Warhammer 40,000 Q&A 3rd Edition
A one-stop site for veteran and power players

The Warhammer 40,000 Webring
Tons of 40k-related links