Connect with players, other GMs, and other fans of the 40k Universe.
The Official Forums
Each game has a set of official forums on the Fantasy Flight site, including subforums for GMs. Tons of great advice, house rules, adventure ideas, world discussions and more are packed away in here.
Dakka Dakka Forums
DakkaDakka is a large, independent wargaming community that features discussion, tutorials and images for many games. They also cover the 40k RPG games. Some great resources crop up there.
Warseer is another large wargaming community with a focus on Games Workshop products
Heresy Online Forums
Heresy Online is another large wargaming community with a focus on Games Workshop products. They cover 40k RPG among others.