When you bite into a perfect summer tomato, do you wish you could have a hundred of them? 

In fact, tomatoes aren't hard to propogate from seed.  All you have to do to create another generation of the best, juiciest, sweetest tomatoes from this year is harvest the seeds, then ferment, dry, and store them for next spring.  Here's how:

1. Use a spoon to scoop the seeds out of your tomato and into a small container.  An empty spice jar, mason jar, coffee mug, or bowl will work fine, and it's okay if you get some of the pulp along with the seeds. 
2. Cover the seeds with an inch or two of water.  Most of the seeds will float.
3. Put the container, uncovered, in a cool corner out of direct sunlight.  Let it sit undisturbed until a skin of mold forms on top of the water.  This indicates that the tomato seeds have fermented, just as they would in nature as fallen tomatoes decay on the ground.
4. Skim the mold off of the water, being careful not to remove any of the seeds. 
5. Add a bit more clean water, if necessary, and shake the container to agitate the seeds.  Then let them settle, and remove any pulp and seeds that float.  These seeds have either not fermented or are duds, and should be discarded. 
6. Drain the remaining seeds by rubber-banding a piece of cheese cloth to the top of your container and letting the water drain out, or by dumping them in a fine-mesh strainer.  Pat them dry with a paper towel.
7. Spread the seeds out on a plate or paper towel and put them someplace cool and dark, where they will be out of direct sunlight and undisturbed, to dry. 
8. When the seeds are dry, store them in an envelope labeled with the type of tomato you harvested them from.  Come spring, you'll be able to grow all your favorite tomatoes again!