I'm gonna go ahead and say there's four that are tied for first place, since the thought of picking just one of these is agonizing:
1) Biking and/or walking everywhere within a 25 mile radius. My wife and I have been doing this for five years, after our car was stolen and we decided not to replace it. Cash in the bank and a great cardio workout. What's not to like?
2) Turning pallets and other people's "wood waste" into useful and badass things, e.g. furniture, fences, shelving units, hat/clothing racks, workbenches, cabinets, a bike trailer (used conduit piping for this one, too) et alia. If we were to buy new or used all the stuff I've made, it would've cost us at least a grand. This will be a major money saver going into the future, especially as my skills refine with further experience.
3) Cooking simple homemade really tasty vegetarian meals. The simplicity keeps our grocery costs quite low and the tastiness strongly inclines us to prefer homemade meals to ordering out.
4) Keeping a regular fitness regimen that doesn't require a gym/service fee. It's hard to calculate exact savings, here (excepting the annual $400-600 gym fees), but considering counter-factual scenarios in which we didn't exercise, it's clear that we've avoided (and very probably will prevent) facing many health-related ailments & the high costs that come with.