Within the USA

Baker's Creek Heirloom Seed
Double Helix Farms
Gourmet Seed
Homegrown Tomatoes and Peppers Home Page
Johnny's Selected Seeds
Marianna's Heirloom Seed
Olde-Tyme [Heirloom] Seed Company
Pinetree Garden Seeds
Sand Hill Preservation Center
Seeds of Change
Southern Exposure Seed Exchange (organic seed source)
Territorial Seed Company
TomatoFest
Tomato Growers Supply
Tom Wagner Seeds
Totally Tomatoes
Victory Seed
Wild Boar Farms
Note: several of the above seed companies also have commercial seed departments for larger orders

Seed Resources Outside of the US

Seemnemaailm
Tatiana's TOMATObase Seed Catalog
The Diggers Club

Some Seed Exchange Programs

Good programs and resources for locating heirloom seeds

Seed Savers Network (Australia)

Heritage Seed Program (Canada)

The Seed Savers Exchange (USA)

web based seed exchanges not limited to tomatoes

Garden Exchange at Gardenweb

Links to obtain plants

Laurel's Heirloom Tomato Plants

Selected Plants - Speciality Tomato Seedlings

Variety Lookup Resources

Tatiana's TOMATObase - a wiki-based resource for looking up information about many different tomato varieties.

Tomaten-Atlas A German langauge based tomato atlas of varieties.