Weapons
In TKD I was shown the use of a knife, it was a formal kata that used rolling and checking actions (useful) with some kicking (not so useful). A friend who was in Ishin Ryu also taught me what he knew of proper use of nunchaku.
In karate I was shown the use of the Bo (two kata) and one kata for Sai. I never took Sai seriously. Bo was more useful.
The use of weapons only strengthens your karate, the use of proper stance and footwork is accelerated in weapons work. Weapons can kill you much more easily than empty handed work (wood and metal are always harder than bone and skin), so you learn to move more smoothly and nimbly as a result. If you have been shown proper dynamics, the transition between empty hand and weapons is fairly easy. If your stuff is too square and stiff, then the use of weapons will be foreign and will conflict with your karate.
As for weapons on your person, I think that one should try to always have a good ball point pen, a pocket full of change, and/or a small legally sized folding blade. These three items are great to have in a variety of situations that range into the upsetting and nasty variety. Some friends of mine also fancy the Surefire tactical flashlights.
Glenn R. Manry
---Iaijutsu, don't forget the doorman.