Tool storage, in there I have:

A spare crank position sensor
Tire 'worm' plug kit
flat blade screwdriver
#2 phillips screwdriver
adjustable wrench
1/4" drive metric socket set
... a few "star" bits
spare bulbs (corner, headlight, tail/stop/turn)
Roll of Duct tape
Roll of electrical tape
wire strippers
wire crimpers
assortment of electrical lugs/terminals

It also serves as a "leak detector", if you find water in there, you know you are close to flooding the fender wells and seeing the end of your top module and security system module/lock pump.