Heirloom seeds are seeds that haven't been bred commercially. Many of the strains were grown the same way in the 1800s.
[link|http://www.seedsofchange.com/garden_center/product_details.asp?item_no=S11151&UID=|Heirloom tomato seeds] - these are the ones we planted a few years ago. The Brandywines are phenomenal - big (1# or more), sweet, juicy, meaty. Eat them like a coreless apple. :-)
[link|http://www.heirloomseeds.com/|Here's another place] that sells heirloom seeds.