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agreed that a small SAK classic is the way to go. Small blade, file/flathead, scissors (perfect for those pesky threads), tweezers, and a toothpick for after dinner.
Plus they come in a gazillion different colors and patterns, so with a little google search through different vendors, you're sure to find one that speaks specifically to you. Shoot, get one for your wife too!
Coupled with a cheap .50 cent carabiner from your local hardware store and you're good to go. I don't leave home without it.
My 2nd choice, which I keep one on my spare key is a Leatherman Style;
The PS comes with tiny pliers and scissors folded into the handle, (no knife); but as I see little value in pliers that small, I prefer the CS model which has scissors instead of pliers, and a small knife folded into the handle. File/flathead and tweezers the same, just missing the toothpick, but.... the small carabiner built right into the handle doubles as a bottle opener, which is a nice touch.
Both are awesome little keychain options, small enough and light enough to not really impede with anything, you barely know they're there, but in times you need a quick cut, and don't want to frighten the locals or cause a scene, (or have someone else cause a scene because they think you're about to stab your wife or kid with a 3.5"+ blade), they really do come in handy.