There's still (a little) time to save seeds from your crops that you intend to grow again next year. This year's favorite tomatoes are a priority for me right now. Normally, I like to save seed from the first and largest fruits as well as from tomatoes on any exceptionally prolific plants.
To save seed from tomatoes:
1. Pick and wash fully ripe fruit.
2. Cut open the tomato across the equator (not end to end) to expose the most seeds.
3. Scoop out the seeds and pulp into a small bowl.
4. Cover with water and let sit at room temperature for 2 to 4 days.
5. Once a day, pour off the top water, pulp and floating seeds. Replenish the water.
6. The seeds are ready when the water-pulp mixture appears to be "fermenting" (see photo).
7. Carefully pour off the top water and pulp one last time. The viable seeds are at the bottom.
8. Drain in a fine sieve, rinse and shake off water.
9. Dry on plastic, ceramic or glass plate and store in a dry place. I put mine in an envelope and place all the envelopes in a kitchen zip-lock bag. Alternately, dry on newspaper and store the seeds with the newspaper (they'll be glued to the paper). To plant, tear off a piece with the seed on it and plant together.
Seed will remain viable for 4 to 10 years, depending on the variety.
Now, I am assured of having seeds from favorites to start next spring!