Harbor Freight paint sprayer.
Although technically I've used it three times, to paint my house, garage door, and deck.
The thing worked great, and having actually followed the cleaning instructions and used the proper storage fluid, I can be pretty confident it'll work again if I ever need it.
Speaking of paint, I stopped bothering with cleaning paint rollers long, long ago. It's well worth spending a few bucks on new ones with every job. I'll put a sealed bag or a cover over a wet roller for continued use the next day, but washing all the paint out of a quality roller takes absolute ages, hundreds of gallons of water, and besides you never actually get all the paint out. So that's more a disposable tool than a one-time use.
As the mechanical tools, I got pissed off and bought a hydraulic press to do a wheel bearing after Numpty McNumbNuts at a local machine shop ignored my explicit directions and markings and pressed one in backwards, ruining an $80 bearing (and then wouldn't even refund the $40 for the service). I thought it would be one of those one-time tools, but it's actually been extraordinarily useful.
Another tactic is the buy/sell -- I once bought an engine lift, used it for a few weeks for a project, then sold it on CrackList within minutes for $40 less than I had paid. MUCH cheaper than renting. I wanted to keep it for assorted hoisting duties, but it took up way too much room, even folded up.
Like a fire extinguisher in many respects. We have great luck with decarbing the engine then popping those plugs out with a 3/8 cordless impact.
