, $15 - a fairly ordinary low-end single AAA LED flashlight, about 10 lumens give or take - not a powerhouse by any means (Fenix sells some single AAA powerhouses, but they're quite honestly overkill), but quite adequate for just about every situation where a keychain flashlight is useful. Feed it with Energizer Ultimate Lithiums, and you get about 16 hours at full brightness, and about another 10 hours of diminishing but still useful brightness. I give it a battery every 6 months to a year, and I've had it for 5 or 6 years now.
I also carry a Victorinox Rambler
on my keychain, but that's not exactly cheap, although I certainly didn't pay $33. (I could've used a Manager instead of a Rambler, having a pen is honestly more handy than the crappy toothpick that the Rambler comes with, but eh, no biggie.) Use the damn thing every day, though.
The Husky 8-in-1 screwdriver
is damn useful, especially when working on laptops or other small electronics (and they have a Torx version too). The bits corrode a bit easily, but it has a lifetime warranty, and it's friggin' $6. Quite a bit more comfortable than the "nicer" precision screwdrivers I've used, although the bits don't bite quite as hard as those nicer ones.