I'm always carrying my Leatherman Squirt P4 micro multitool (pliers). Very well built, very small and light, has all I need to maintain computers at work, and for everyday needs.

Another knife which is very personally crafted, compact, and _truly_ always with me (yes, even in the bathtub!) is my 3/16" long blade thumb-nail. Properly shaped and properly sharpened every other day - it is perfect for opening mail and cardboard/plastic packages, cutting strings and tapes, opening latches in plastic laptop cases (and other electronics) without damaging its finish, and also works as a quick PDA stylus substitution.