Mine are probably a small pair of pliers that I use a lot for removing and adding spikes, tinkering with track, etc. I also have 1-2 very small screw drivers that seem perfect for modeling -- and even scraping away excess ballast & adjusting track.
I also have a small plastic tool box (with various compartments and slots) that contain my spikes, track cutting saw, pliers, screw drivers, fishplates, etc. I got this from Home Depot and it's great. It helps to keep me organized & I need all the help I can get with that!
Rob