Simply because of the sheer ammount of tools and parts I have I split it all into three seperate kits. The one I leave at home for maintaining everything, the one I take to the field and a seperate kit for Creed, Ion, Ego 8 and Halo parts. In my home kit I've got:
Chronograph < - - - Best thing to have if you're going to be in paintball for any decent length of time. Makes teching so much eaiser.
Allen Key Set
Various Screwdrivers
Stanley Knife
Super Glue
Epoxy Glue
Fabric Glue < - - - For loose goggle foam.
Micro-Fiber Cloths
Lube
Loc-Tite
Macroline & Banjo's
Screw Removal Kit < - - - Because nothing ruins your day like a sheared screw.
Needle Nose Pliers
And in my field kit it's:
Allen Keys
Electrical Tape < - - - Always handy. Battery door falls off? Tape. Loader loose in the feedneck? Tape. The screw that attracts the trigger magnet on your Ion falls out? Stick an Allen Key in there and Tape.
9v & AA Batteries
Screwdrivers
Lube
Small Spares Kit for my Marker
Cat Crap Anti Fog < - - - Always good if you go with friends to a rental day. Keep their googles fogging.
Microfiber Cloth
Adjustable Wrench
Stanley Knife
I admit that most of it is far from essential but It's nice to know it's there if needed