Tomato, cucumber, etc seeds - I scrape out the seeds with a spoon, put into a shallow dish. Add a few teaspoons of water, set in a warm spot for several days. Once a mold forms on the seeds, I wash them in tepid water until the pulp is washed away. Then I spread them on a coffee filter to dry. Store them in a paper envelope in a cool dry place. The mold forming kills any viruses present in the seeds, and using a coffee filter prevents the seeds from sticking fast like they will if paper towels are used. Works with most fleshy pulp bearing seeds.
Oh yeah, try to find heirloom seeds for anything you want to save seeds from. If you start with a hybrid, the seeds you save may not give you the same produce the following year.