I bought heirloom tomato plants this year specifically so I could save seeds. The cost of a tomato plant is between $2-5 at the farmer's market for heirloom tomatoes. Each plant produced 5-10 pounds of fruit. That's a pretty good deal, but an even better deal is to save the seeds and start again next year from seed! I will be planting my tomato starts as early as February this year-- much later, and they don't get big enough in our climate. I was lucky I got any ripe fruit at all this year!
Here's what you do to save seeds:
Let a few tomatoes over-ripen and get all squishy. Then you cut them across their equators and scoop out the seeds. Put the seeds and the goop around the seeds in a jar along with a few tablespoons of water. Put plastic wrap over the top and poke a hole in it, to allow air circulation. This water will get scummy, so change it every day until the seeds are naked. Then, lay the seeds out on a paper plate or coffee filter to allow them to dry. Once they are bone dry, you can put them in an envelope and save for next year.
You want some of that, don't you? Mmmmm, good.
People who know a heckuva lot more about seed saving are also on the web. Check 'em out!