Here are some of the most popular aquarium forums. It is nice to be able to interact with other people and get feedback or advice. Being involved with a live community also keeps you up to date with the trends. They are also a good resource for saltwater aquarium product reviews.
You will also find that people share pictures and videos of their aquarium set up. This can be very help for anyone researching or starting a new aquarium.
Reef Central is probably the largest online reef aquarium community. A good forum for new and advanced hobbyists.
The Reef Tank is another large forum, with a nice crowd. Overall, they are very helpful and have many experienced aquarists.
Reef Lounge is one of my favorite forums and was one of the first places I registered when I started my saltwater aquarium. They are very good with beginner questions and it is the best forum for people just starting the hobby.