It's maybe not what OP had in mind, but I feel training my dog definitely fits here. A trainer would charge a couple wheelbarrows of cash to train my little guy. I do it myself (probably slower), and save quite a bit. He knows most of the "basic" commands, and I'm currently working with him on his pulling (he pulls against a leash like he's being paid for it, literally to the point of choking himself). He has improved dramatically in this regard since I first got him. I also taught him to shake hands. Haha. As for resources, I really like Ian Dunbar's stuff. He's a British veterinarian and dog trainer. Reddit also has a dog training community.
I also do most of my own car repair, except for oil changes. I can almost always find a coupon for Car-X so that it ends up being cheaper than doing it myself. I also recently discovered that you can use the metal cleaner "Brasso" to clean foggy plastic headlights. Often, I buy my parts on ebay using an ebay gift card that I purchased at Kroger. That way, I get (double!!) fuel points for buying parts I would've bought anyway.
Computer repair! This one has saved me a ton of money. A technician has never touched any of my computers (I have a desktop and laptop, both Dells). I'm very good with computers and have no issue whatsoever repairing them. My CMOS battery recently died (the battery that allows the BIOS information to save, and powers the internal clock) and I replaced it myself. I've also had other hardware (and software) issues that I have troubleshooted myself and solved, typically for nothing. My laptop was starting to get hot enough to burn me, so I disassembled it completely, and discovered that there was a wall of dust between the heatsink and fan. I removed this, and it did help quite a bit with the temperature. However, I still need to apply some thermal paste to the CPU and GPU. It's slightly related, but I have an "old" iPod video (5.5 generation, I believe). It stopped turning on. When I tried, the hard drive would just click repeatedly until it shut off on its own. This ran the battery down. I took it to the Apple Store and one of the "geniuses" (I assume they use the term ironically), after just a visual inspection without hooking it up to a computer, told me, "It's broken. Buy a new one." I lol'd and left. I use Linux, so at first I wasn't able to install iTunes to fix it. Finally, I figured out that I can install Windows via virtual box and install iTunes in that. So I did. I was able to restore the iPod, which fixed its "clicking" and not turning on problem. However, the battery was still dead and would no longer hold a charge. I bought a new battery on ebay for $1.99 and installed it myself. My iPod is good as new.