[Travis L. Stevens]

I haven't been able to find anything that beats the Australian Marine Institute of Science's Coral Search page. While the page never seems to work for searching, all the results work fine. *Note: This site is only for reef building corals. No softies.
http://www2.aims.gov.au/coralsearch/coralid_search.php

These are my personal recommendation for references. There are LOTS more, but I can't remember them.
Books

Aquariums
The Conscientious Marine Aquarist
The New Marine Aquarium
The Complete Book of the Marine Aquarium
Book of Coral Propagation

Fish
Marine Fishes
Reef Aquarium Fishes

Corals
Aquarium Corals
Corals: A Quick Reference Guide
Corals of the World (Vol 1, 2, & 3)
The Reef Aquarium (Vol 1)
The Reef Aquarium (Vol 2)
The Reef Aquarium (Vol 3)

Invertebrates
Marine Invertebrates
Invertebrates

Online Sources
Reef Central
Marine Depot Forum
SeaSlugForum.net
SeaHorse.org
FishBase
AlgaeBase
AIMS Coral Search
WetWebMedia