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.