Below are links to various manufacturers, suppliers and useful reviews.

Adorama
Adorama has nearly everything for the photographer, with excellent prices and fast service

B&H Photo Video
Like Adorama this is a one stop shop for photography supplies

Photozone
Photozone has accurate, fair reviews of lenses.

Ken Rockwell
Mr. Rockwell maintains an extensive website with excellent reviews and tutorials. He has a somewhat variable reputation on various photography forums, but I have found his lens/camera reviews very helpful.

Steve's Digicams
If you want information on the latest digital cameras, this is the place to go.

Savage Paper Co.
Manufacturer of photography paper backgrounds.

Expo-Disk
A valuable aid in setting the white balance on a digital camera

JTL
Slave Strobes

Gray Card
High quality gray card

Smith-Victor
Manufacturer of lighting accessories

Hoya Filters
Manufacturer of high quality filters such as polarizing filters

Eiko light bulbs
85 watt 5000K fluorescent lamp

Westinghouse
27 Watt full spectrum fluorescent lamp

General Electric
500 Watt 3200K Tungsten Lamp (color temperature not important for digital photography)

Bogen/Manfrotto
Manufacturer of quality tripods and lighting supports

Lighting tutorial
Mostly devoted to portraiture lighting, but still informative.

Adobe PhotoShop Elements 7
An inexpensive, yet powerful photo editing program