Hi all, it is time that I update this and give it a new face. I want this to be more of a resource, so I will keep updating this top post with the most useful links, this way you don't have to scroll through all messages to find the links you need. You are welcome to post more and I will add to the list, but I will try to keep every link in this first message. So here is a compilation of what I consider the most useful and user friendly ones (the ones in bold are the most scientifically accurate, the others have mostly good IDs but they need to be double checked):
Fish:
FishBase http://www.fishbase.org/
Australian Museum http://www.amonline.net.au/fishes/identify/
Fishes of the Tropical Eastern Pacific http://www.neotropicalfishes.org/sftep/
Gobies http://www.gobiidae.com/
Fish Photo Database http://www.fishdb.co.uk/
Invertebrates:
Sea Slug Forum http://www.seaslugforum.net/
Nudibranch Identification http://www.nudipixel.net/
Mediterranean Slugs http://www.medslugs.de/
Cephalopods http://www.cephbase.utmb.edu/
Crustaceans (links) http://www.vims.edu/tcs/crust_links.htm
Both:
Coral Reef Creatures http://www.reefimages.com/
Marine Life of the Philippines http://www.poppe-images.com/
Flora and Fauna of South California http://www.ladiving.org/fieldguide.html
SeaLifeBase http://www.sealifebase.org/