Just bring a few spare links, and the tool to press a broken link out and replace it. I've bent a couple links mountain biking. Usually I can muddle through and change it when I'm done, but once it started jamming and I had to replace it on the trail. It only really takes about 5 minutes.

Another thing I bring is spare brake pads. On a very long downhill once, my brakes started to fade, and when I got to the bottom I stopped and looked and the rubber brake shoes were melted all over the rims. I guess that's why some guys like disk brakes!