Typically as people learn to fix things about their bike, a few things still usually mean a trip to the bike shop: bottom brackets, headsets, hydraulic disk brakes. Every bike has the first two of those, and my mountain bike happens to have the later. (Thankfully, they've yet to need to be serviced.) There are two reasons that people stay away from fixing bottom brackets and headsets: One, they are scary and important parts of the bike and doing something wrong with them can cause you bodily harm while riding or damage to your frame. Two, they need their own special tools.
I've need to do a lot of bottom bracket replacement on my bikes, both replacing old ones and moving ones between frames so I picked up a Park Tool BBT-22 and stick to bottom brackets that this tool will work with. Everything I ride is square taper, and the only bottom bracket that won't work with this tool is the on in my track bike due to it's adjustable chainline (this BB needs it's own special $50 tool but it should stay in that bike for a while). The BBT-22 is only about $20 and gets the job done pretty easily.
Headsets however, are a lot more trouble. There is one kind of tool to remove them, and another kind of tool to install them. Neither of which I had or wanted to pay for because I almost never need to do this kind of work, and buying tools for these things is just silly. This was fine until the headset on my mountain bike started to feel sticky. I pulled the fork out and found out that unfortunately, the headset was a semi-cartridge headset. These are better than cup and cone headsets because they don't require adjusting to work correctly, but worse than cartridge headsets because grit and water can still get into the bearings pretty easily and to repack the bearings, you really have to remove the entire headset and soak things in degreaser first instead of just popping out the bearings. This meant it was headset tool time!
I didn't want to buy big expensive tools when other things would do... For removing headsets I used a piece of copper pipe and a hacksaw per this example. $5 for the copper, and with a few smacks with a hammer my headset was out. I cleaned things out with WD-40 until both the top and bottom bearings spun smoothly with no gritty sounds, and repacked them with grease. Then for re-installation, a rubber hammer was all I needed! I put the opposite side of the frame on a wooden block outside, put in the headset, and just gave it a good smacking and that was it. Some people say that this is not enough to get a headset fully seated, but it's close enough for me. With a press like the park tools one, it is possible to over-tighten a headset and damage it or your frame, so usually even with these it remains too loose and takes a few miles on the trail to get fully seated anyways. Hopefully I'll go riding tomorrow and will get to test it all out!