Saw a variant on this question a few months ago... It really depends on what type of work you do.
A lot of conduit? Well, then, a few boxes of 1/2 & 3/4" connectors, straps and couplings.
If you're in a NM area, the EMT stuff would sit in the back and take up weight, but I bet you'd have a box of "Hit-Locks", where I wouldn't...
Service calls? Drywall compound/spackle and "cut-in" boxes, as well as FMC/"Greenfield" (or NM), as well as a small assortment of bath fan/lights, basic lighting fixtures, etc.
New construction? Cases of 1900's (4" sq. j-boxes), bundles of sticks (or coils of NM)
It goes on and on... you really have to look at the kind of work you're doing (and the mats you're using) over the last ten jobs or so and make up a list for your particular situation.
[This message has been edited by DougW (edited 11-22-2004).]