If you find yourself constantly needing a bit driver, you can build a keyring version by adding a magnetic bit driver designed to be used with a power drill to a carabiner keyring. Simply attach a nut large enough to fit the carabiner to the end of the magnetic bit driver with a little JB Weld.
Instructables user uglymike figured out the method to attach a magnetic bit holder to a carabiner keychain. Once that's done, just add a set of keychain driver bits that are available at most hardware stores and you'll always be ready to tighten or loosen a screw.
Build a D-Ring Bit driver [Instructables]