Glow plugs, spare receivers, allen wrenches, pliers, screw drivers, super glue, electrical tape, double stick molding tape, on/off switches, lots of wire ties, glow plug wrenched, spare batteries, jewelers set of screw drivers, small files, and right now the whole box of stuff hasn't been outside in a year.
With allen wrenches, don't go cheap. MIP are probably the best out there. I use the MIP speed bits in a 1/4" electric driver and I keep a set of losi hand wrenches in my pit bag for when I'm out bashing. I've had the losi wrenches for 15 years. Totally worth the money. the MIP's have held up for 4-5 years so far. I recently ground a small hair bit off the end of my 2mm wrench as the very tip was a bit rounded.
Otherwise, a small pair of channel locks come in handy, needle nose, side cutters, small tuning screwdriver, small vice grips are things in my pit bag.
I have a small parts organizer I keep in my truck bag as well that has wheel nuts, wheel hexes, exhaust gaskets/couplers and header springs, various ball ends, spare nuts/bolts/washers in various sizes.
Half of my home office looks like a hobby shop... never went so far as to put parts on a peg board like I've seen a few others do, but I have multi-drawer cabinets full of parts, old engines, pipes, chassis's, wiring etc. I also have 4 other small parts boxes, one for all my bearings, one for all my engine/cb parts (shims, bells, clutch shoes, flywheels), one for all my odds/ends, various ball ends, bolts, screws, etc and one for electrical stuff, wires, ends, shrink wrap, switches, plugs... 20 years of RC and I accumulated a lot of stuff. I need to go through my drawers and make some room for arrma parts... Now that I have an arrma truck, I need somewhere to keep them separate. I have a drawer for traxxas and another for HPI, need an arrma drawer.