Clothes appropriate to the weather! (Many people forget this, it can be the single most important thing, depending on your location.)
Wallet (car insurance, registration, and medical card(s), at a minimum, in addition to what LedTed already mentioned).
Fisher Bullet Space Pen (titanium nitride).
Kershaw Blur (no serrations, drop point).
Keychain (apartment and car keys, and a small light).
Cell Phone.
Zebralight SC600 (not strictly necessary, but very easy to EDC, so why not?
This is the minimum I carry if I am going to be out for a while, because it is enough to deal with virtually any situation, even if you are gone longer than expected, or have an accident, etc. If I'm just walking around the neighborhood, I might take only a couple things. At night, keys/flashlight/knife is adequate (given good clothing). During the day, just the keys and knife will do, if I know I'm not going to be out for long.