I like to keep my AR ready at all times in case of... well, whatever.  I shoot about 500 rounds a month.
What part(s) might/will wear out that I should have on hand for replacement.
I'd hate to be in a situation where my gun is rendered useless because of a broken/worn part that I can't change.  

Here is a list of what I keep.
firing pins
firing pin retaining pins
extractor
extractor spring and insert and ret. pin
ejector
ejector spring and ret. pin
cam pin
several roll pins
front sights
all the springs in the gun
bolt catch
and anything else I think that might break under extreme use.

Roll Pins can be obtained at hardware stores, and cut to length if necessary.  The springs, you MUST have the right springs, and they are cheap.

A bolt stop assy and mag catch assy are good to have, as is a complete bolt.

I see no need to keep a spare forward assist.  Even if it broke, not much of a loss, is it?  You could always plug the hole with a blob of silicone or epoxy.  Or remove the pawl and put the plunger back in.

McFarland Gas Rings.  Put one in your rifle, and one spare, and you will likely never need the spare.

Hammer, its spring and pin.

Trigger, its spring and pin.

Disconnector, its spring.

ditto what those guys said.

something i've learned.

you can't have too many lower reciever parts, especially springs.

colt

I just keep an extra AR (or two) around.

Well, if this national ballistics fingerprint database thing ever happens you might want to consider a spare barrel or two, bolts, firing pins, extractors and anything else that leaves an identifiable mark.