I save seed from any open pollinated plants in my garden. Right now, the lettuce is going to seed. There is a jungle of dill weed. And I harvested about 4 oz. of kale seed from the green house. Bean seed is an easy one, b/c they are self pollinating. Tomatoes will more often than not breed true unless they are hybrids. And many folks even save hybrid seeds b/c the next generation may yield a fruit that is similar, and there just might be some pleasant surprises. One year, I planted seed from a 22# giant buttercup that I bought at a neighboring farm. That fruit yielded similar size: green buttercup. That year, it crossed with my Red Kuri. the following year, I had 22# giant red/orange buttercup, some of it was red with green tiger stripes. Unfortunately, those hybrids reverted to red kuri the following year. Suyo long cucumber is an other excellent OP. Parsley will self sow if you allow it to go to seed the second year. If you allow the scapes on garlic to mature, they will produce loads of bulbils that will eventually produce more garlic than you could ever hope to use. I sow bulbils in the orchard, and give a lot of them to the chickens. Nasturtium, calendula, zinnia, marigold, and many other flowers are great for saving seeds... or simply let them self sow.