Fish keeping Forums:

Aquatic Hobbyist Sections for Tropical, Marine, Plants and so on. Very friendly board where the newbie is assured a warm welcome. Good articles and expert advice.

Reef Central Probably the largest marine forum in the world.  Loads of information from some very experienced hobbyists and professionals.  Possibly a little "American"  but the majority of the advice is just as valid in the UK.

Tropical Fish
Aquarium website that features a large number of articles, species profiles and plant profiles as well as a photo and video gallery, forum, chat. Aquatic Community offers all visitors a free forum.

Ultimate Reef Probably the largest UK marine keeping forum. Very handy for a UK perspective: livestock, treatments and equipment that we can buy here.