This is a good thread topic! My addition will be the thermarest patch kits. Obviously they are good for patching your Therma-rest (at least I hope so; never used it for that), but the epoxy glue is good for fixing hiking boots. Many of the newer boots have moulded bottoms with different types of rubber glued together. I have seen these come apart quite a few times, and I have put them back together with the epoxy in the patch kit. The epoxy is good for putting quite a few things back together and I always bring the kit now.

Another; forget or break your tent stakes? Take your pocket knife out and make a set and/or use rocks.

I always bring needle and thread, couple of buttons, and safety pins.