You'll probably attach the Wishbone Wrench multi-tool to your key ring, then forget about it. But you'll be happy to have it when you need it.
About as small as a key, it sits unobtrusively on a ring, but has 10 different tools, including multiple screwdrivers, rulers, wrenches, a file, and, most importantly, a bottle opener.
$10, kickstarter.com