digital

Phil Askey's UK based website is the world's premier digital photography website.   It has detailed reviews of virtually every digital camera which has been produced, and very active forums for different cameras and interests.   The Learn Digital Photography section is the weakest part of the website, you're better off going to Luminous Landscape if you want to learn different digital techniques.

Luminous Landscape

Michael Reichmann's "site devoted to the art of landscape, nature and documentary photography" boasts that it has 1,900 advertisement free pages.   Perhaps the most useful are the "tutorials" and "understanding series" pages selected from pull-down menus at the top of the home page.   These sections have excellent ideas on various topics related to digital photography, not just to landscape and nature photography.

insects

How to Photograph Dragonflies with Flash

This page by David Westover resides under the auspices of the Michigan Odonata Survey website.    It's pretty detailed, covering topics like the camera body, what lenses are appropriate for dragonfly photography, and how to use flash unobtrusively, and there's a very good gallery of dragonfly photographs he's taken.

underwater

Ikelite	Ikelite is a highly respected company which manufactures clear polycarbonate housings for both SLRs and point-and-shoot cameras, and also manufactures their own strobes.
ewa-marine	This company manufactures very popular flexible plastic housings for underwater still cameras and video recorders.   They're cheap, but I'm sceptical about their value to serious underwater photographers.

commercial

The stock photo price calculator page on this website gives a range of prices for various types of photography, based on surveys of the prices charged by commercial photographers.