Here is a list of some great resources that pertain to the hobby.

Calculators

Reef Chemistry Calculator – JDieck – Ca, Alk, Mg, affects of additive brands etc.- 
Calcium Reactor Calculator -- JDieck -- How to dial in a Calcium Reactor
Water Volume Calculator -- JDieck -- Type in your dimensions and see an approximate total water volume
Water Change Calculator -- Hamzasreef -- Type in your dimensions and see an approximate total water volume

Coral & Fish ID

AIMS Coral Identification -- Great resource to help identify corals
Fishbase -- Database of information on Marine Fish
Algaebase -- Database of information on algae

Reef Forums

Reef2Reef -- Everything under the sun that's reef related can be found here
Reef Central -- Another forum with great information. One of the original Marine Aquarium hobby forums