+1 on the Spyderco Ladybug. I keep trying other items but come back to it. It's just big and sharp enough to be useful, but unlike the Leatherman miniatures, it doesn't bulk up in your pocket to an annoying extent.

Note that I mount my Ladybug on two stainless split rings (bead chains tend to fail) which are then put on the main ring; this lets it twist around so that it lays flat no matter what. The keychain acts as an extended handle.

I also have a thin all-metal Gerber in 440x that cuts well and weighs nothing, but it's a bit longer and I find that it twists and jabs more than I care for. YMMV.