Here are a few of the many great on-line resources devoted to heirloom plants:

Seed Savers Exchange is an excellent resource for gardeners interested in heirloom plants.

Garden State Heirloom Seed Society is a great local seed saving organization based in New Jersey. It was founded by Joe Cavanaugh, an incredibly active heirloom seed archivist.

Tom Clothier's Garden Walk is a very informative site regarding gardening, botany, and seed saving.

Native Seeds/Search is dedicated to the preservation of mainly southwestern native flowers and vegetables.