This sort of thing is very specific to personality types. For my personality type I've found the following list to save me huge amounts of money. I'm in South Africa, though, so depending on your circumstances some of these may not make sense to you.
1. Automobile diagnosis and repair: A repair (water-pump, thermostat, cambelt and everything in that work-path) was quoted at R19k (ZAR) from the dealership, I bought the parts from them for R8k and did the work in a weekend. Generally, if you can do your own maintenance on your car, you have no need to upgrade it every time you pay it off. By my rough calculations, by making our cheap cars last 350000km or more, I've saved enough to buy a small flat[1].
2. Basic metal-working skills: You'd be surprised how many problems round the house are solvable if you can weld one piece of metal to another. A lot of the fancy tools that cost big money (like the camshafts alignment tool, which costs around R2k) can be solved by simply fashioning a piece of metal to do the job.
3. Plumbing. Learn the basics of fixing leaks. A callout from a plumber, without any work being done, is R1.8k. I recently fixed 3 major leaks with around R200 in parts (pieces of pipe, ferrules, etc).
4. Basic carpentry: This is a nice to have, but almost never saves me much money. I may save a few hundreds for shelves, cupboards, but the time investment is way more than I like considering how little I save.
[1] My house is close to being paid off. I credit this with the fact that our three cars only cost insurance and repairs, and so I could put cash into the mortgage that other people put into the purchase of a car. This means you have to be willing to own the car, not merely rent it until your next 'upgrade'.