King Arthur Flour
http://www.kingarthurflour.com/
King Arthur Flour is where we took our class on hearth oven baking. We also use a culture that we brought home from their Vermont workshop.

Bread Bakers Forum
http://www.angelfire.com/ab/bethsbread/FrontPage.html
This site seems to be a good resource on bread baking and sourdough. The sourdough section provides a good list of terms that are used in discribing sourdough baking or working with natural leavens.

Sourdoughs International
http://www.sourdo.com/index.htm
This site is run by Ed Wood, author of World Sourdoughs From Antiquity. Sourdoughs International is a company that sells a variaity of sourdough cultures from around the world.