Hobby guns? Nothing.
Serious-use handguns? Aside from keeping an exact copy as a (reserved for) defensive-use -vs- training/practice duo, I have enough spare parts to rebuild the entire thing, minus the barrel, slide, and frame (hence "an entire spare gun" idea, borne of the words of Ayoob and Hilton Yam). I also stock recoil springs, striker springs, and magazine springs in order to service the weapons, but in terms of the magazine spring, I do treat the magazines themselves as disposable, and have accrued what I believe is sufficient number of them to do so, for my conceivable lifetime.
My defensive and training ARs are the same. Again, my main goal was "to have a spare," but once I achieved that, I started accruing parts per the advice of some of the sources that I respect (such as the late William Larson, Chad Albrecht, and also the late Pat Rogers and his many disciples who are local to my area: more on this in a minute). Outside of the barrel and upper/lower receivers, I have enough to overhaul the entire gun, for every one I have. As with
@TEXASforLIFE , I also keep spare BCGs and bolts, and I'll typically check headspace at least once a year to see if the "spare" I'd intended to use for that gun is still good-to-go, with this critical measure. One thing I don't bother with for the AR are replacement magazine springs - the action spring, buffer, and ejector and extractor springs, along with gas rings and cam pins are replaced as service items.
I haven't really delved into the art of the shotgun just yet, so for the time being, I've got just one pump and one semi, and I don't really have much for either, in terms of either service or repair components.
@Wannabewoodsman , there's actually service manuals and other armorer's resources available (for free) online for some of the more popular weapons and platforms, such as the Glock handguns and the AR15. Primary & Secondary, ARBuildJulkie, as well as platform-specific discussion Forums (such as GlockTalk) typically have these resources highlighted, stickied, or otherwise bookmarked.