Reef Central

The largest saltwater hobbyist community worldwide.  Lots of great features including Reefkeeping Online Magazine and incredible aquariums featured for Tank of the Month.

Canreef Aquatics

A Canadian based forum with many quality posts and an extensive reference library featuring dozens of links to informational articles.

TGVAS

Aquarium society local to our location in Victoria, BC.  Great thread on live rock hitchhiker identification located in the Advanced Reefkeeping forum.