So what is the ultimate in mobile tool storage? In our research, the absolute best, most useful, and well-designed thing we've found is this:
I know that looks like the lovechild of a 1980s Xerox machine and an X-Box, but in fact that's a basic building block of the Systainer mobile storage system, found here and here, by German company Tanos. (You may see it cross-marketed as a Festool product, as they are sister companies.)
The Systainer system was designed for people who work with tools both in a shop and on-the-go. There are five basic containers, each one scaled up in size from the one below it on the price ladder.
You can use each box as-is, or fill them with a variety of inserts--pre-formed, user-arranged, or "pick & pluck" foam for customization--to subdivide their contents.
If pull-out drawers are more your thing, there's also a line of Drawer Systainers, again with steadily-increasing capacity.
As for how all of these become a system: See those little green latches? When you stack one of these on another, the latches, rather than holding their own lids shut, slide up and lock to the case above it. This makes it handy if, say, you want to carry a few of them together.
If you then want the entire thing to roll, they make a wheeled base.
The downside to having say, five of these stacked up is if you only needed to access the one on the bottom. For that eventuality--these guys really have thought of everything--they make a Systainer Port, which enables each Systainer to be pulled out like a drawer.
And yes, they're sturdy enough to stand on.
Lastly, so you can get a sense of how these things are made, you can check out this video we found of a guy disassembling/re-assembling them: