Gardening is a good frugal strategy if it's something you enjoy doing. I take great pleasure in gardening, most of the time. When it's hot and muggy and buggy, it's less of a joy. But nothing beats harvesting food you've grown with your own hands.
Seed saving is a great strategy to save money in an already frugal endeavor. There are so many particulars to saving seed from the huge variety of cultivated plants, that I can offer very few how to's. There are entire books devoted just to saving seeds from the plants we eat, and other forms of propagation as well. Sometimes it's obvious when a seed is ready to be harvested if you simply observe the plant closely. Culinary sage, poppies, beans, kale, parsnips and many other plants form seed heads or pods that dry out and release their seeds freely when they're ready. With other plants it's trickier.
Saving seed not only saves you money, but it also acts as a selection process for your own climate zone or even microclimate. You'll only get seeds from plants that have done well right where you grew them, or where they volunteered. Thus, those seeds have an excellent chance of working for you again the next year. If you live just on the edge of where a plant is able to grow, you may even develop your own cultivars by repeatedly saving seed from the plants that have done well for you from year to year.
Yesterday, I harvested seed from some parsnips I left to overwinter and from my beloved kale lacinato. Both of these plants produced seed abundantly for me. And parsnip seed has a very short shelf life. That means I have more seed than I need. A good website I've found to share seeds freely with other gardeners is Gardenweb.com. It's free to register and become a member. Once you do that you can set up an exchange page with lists of seeds or plants you either have to share or are hoping to trade for. No money changes hands for these trades, which are done by mail. Of course, there's always a slight risk that you'll get burned when proposing or accepting a trade. No one can guarantee that the other person will come through with the seeds they agree to send you. But gardeners by and large are honest folk. I've traded about five times through Gardenweb, and have never been burned.
Check out the Gardenweb site, whether you're an old green thumb or a novice gardener. There's tons of information and expertise there, in addition to thousands of potential seed trading partners.