Wargaming in the Age of Fighting Sail
Essentially an illustrated links page with rules, models, documents, and pictures.

Warships1
A monstrous compendium of data on 20th Century warships of all nations. Ship histories, photos, drawings, weapons data, and disclussion boards. An outstanding resource.

Kriegsmarine Encyclopedia
Lots of material on German warships of both World Wars, animated maps of engagements, articles, links.

WWI - The Maritime War
A substantial collection of historical documents, contributed papers, and data. Ships, tactics, history, OB's, ballistics, all sorts of interesting stuff.

Nihon Kaigun
The Imperial Japanese Navy of WWII. Lots of ship and weapon data, some accounts, bibliography, and other materials. The home page tends to show up with only the logo visible in the browser window. Be sure to scroll down instead of leaving the site in puzzlement.

Seekrieg
The homepage for the authors of the ruleset. Free downloadable version of Seekrieg 4, play aids, info on Seekrieg 5, some material on modelling.

Ship Camouflage
An online database of camouflage used by ships of the US Navy in WWII, just in case you want to know how your favorite heavy cruiser was painted in, say, 1943. Painting guides, signal flags and ship call letters, other goodies.