Terrierman is right, and there are open pollinated (non-hybrid) varieties for about everything including tomatoes and corn. The two seed sources I mentioned are 100% OP or heirloom.
There's always the chance of cross-pollination if there are multiple varieties around, but plants that are self-pollinating (tomatoes, peppers, lettuce, beans, peas) don't cross as much. Insect or wind-pollinated plants (corn, cukes, squash, melon, beet, radish) will readily cross and need isolation to insure you get seed that's true to type.
I buy seed, store it in a cool, dry place and can usually get a few years out of it.