This HF soldering iron:
$5 investment has saved me hundreds and hundreds of dollars so far. It can be a little flaky sometimes, and the tip doesn't get as hot as the side of the tip does, but it gets the job done.
I also have another favorite:
Serpentine belt removal tool. It's $17 now, but I think I paid $8.99 for it at the time. It makes pulling back tensioners incredibly easy. No fumbling around with wrenches and trying to fit them into impossibly tight spots. It comes with almost every size you'll ever need, and is nice and thin to fit.
I was quoted $100 to replace my belt. That's over an hour's labor where I take the car. I bought a belt myself for $25, and had it on with this thing in less than 10 minutes.
I've since used it on several other cars, equally as easy. This tool was definitely worth the investment.