Robert's recent Ridgid cordless drill review reminded me of their portable stacking tool boxes, which I've been using for quite some time now.
They have a "base" box which has wheels and a telescoping dolly handle. A medium box with a tray insert, and small box that comes with removable dividers and several separate cube-like cup containers for small parts and fasteners.
They all have these clips on the top which snap into receivers on the bottom which latch them together, tightly. So you can have 3 or 4 stacked together, forming one solid (dare I say, rigid!?) tool chest which rolls around like a dolly.
They will often put them on sale for $99 for all three (buy the large and medium, get the small for free). Since Fathers' Day is coming up, look for this sale soon.
I originally bought one set of 3 when they had the sale. After checking them out, I went back and immediately bought another set of 3. I think I'm up to three sets of 3 along with a couple "basket" versions, which also stack and lock. The baskets are great for throwing in the job-specific extras that you might not use regularly. I use one for the circ-saw with extension cord, blades, rip guide, etc. Then I have a basket for bulk boxes of job-specific fasteners/nails-- you know, the big 5lb boxes of deck screws and nails and such.
I have one of the base boxes for plumbing tools. Another has my hammers, pry-bars, tool belt, squares, stuff for framing. A medium box has hand saws and files, chisels, grinder, cutting stuff. Another medium box has other themed tools that are usually used together or for specific types of jobs. You get the gist.
The small boxes are probably my favorite, though. I have one that I fitted out to hold my Ridgid drill/driver combo with charger and assorted drill bits and driver bits. Another for a huge assortment of fasteners. Another dedicated to a huge assortment of wall anchors. And I'm starting to convert some others to cases for other power tools, like my Fein oscillating tool with attachments, the jigsaw with blades, etc. I'm thinking of making a whole stack dedicated to pneumatic nail guns and trim tools.
They are all labeled with black sharpie on white gaffer's tape. They fit two wide, and 3-4 high in the space behind the back seat of my minivan. They also can go behind the seat of my pick-up truck. When latched together, you only one strap to tie them down.
It's so nice to be able to keep all the regular, commonly used tools in the van, behind a seat and smoked glass (away from prying eyes), and then be able to grab any job specific tool box to throw in the van/truck knowing they will all lock together and not be sliding all over the vehicle or tipping over and spilling.
I haven't even mentioned the obvious benefit of being able to roll 3-4 boxes at one time, like a dolly, into a building, up/down stairs and back out again.
Oh, and by the way... they're water-proof! They have a gasket that run around the perimeter of each lid, so no problem stacking and tying them in the bed of the truck and driving in the rain. I haven't checked whether or not they float, yet, but maybe I will.
All they need now is a version that is an insulated cooler!