I've never seen this presented here, but since this has gotten into improvised weapons I thought I'd throw it out. One thing that is always allowed, even inside a courhouse or an airport, and something everyone carries everyday anyway, is a ring of keys. Keys, used properly can be an effective improvised weapon. To make them effective as such, simply equip your keychain with one of those "around-the-neck" lanyards. At about 18" or so, these offer some standoff from an attacker, and can deliver a painful blow.
Not only can blows be delivered, but should the attacker manage to grab the keys, he can be pulled in close for follow-on blows or off-balancing techniques. If I didn't have my EDC handgun, I would take this over a knive for 99% of confrontations.