For me, much of the joy of gardening is being able to grow food that isn't available at the grocery store, where the specific varieties of produce offered are more likely to be chosen for appearance, uniformity, shippability and resistance to mechanized handling than for their merits as food.
I just received my copy of the 2010 Seed Saver's Exchange catalog and have been flipping through it like the proverbial kid in a candy store. Which of the 37 varieties of lettuce will my family enjoy in our salads next summer? Which tomato, out of the 72 on offer, will make the best ketchup? Should I try growing okra (five varieties)? Purple Viking potatoes?
SSE is a fantastic non-profit organization (or social-profit, as my friend Sarah says) dedicated to preserving our collective genetic food heritage by collecting, propagating and selling heirloom seeds. If you garden, consider checking them out: http: http://www.seedsavers.org/.