Well, here I sit, in Middle of Nowhere, Iraq. Sitting on an old HMMWV seat, in fact. This node is more military related, but it can be helpful to anyone with a bit of imagination to replace some of the items used!
First off, some materials are required: hammer, nails, plywood, and two-by-fours. Other materials are includes as required for the specific seating.
Makeshift chairs are a staple of wartime seating at remote sites. Typically, you can only bring so much gear out at one time. Sometimes, people try to transport extra and useless crap. For instance, a captain's chair or a camp chair taped to the side of their seabag. This is a Bad Idea(TM) - especially when you've also got a pair of twenty-five pound weights in said seabag, and someone else has to carry it to the helicopter. </rant> True story - we wanted to kill the guy.
Anyway... scrap wood seems to be an abundant resource in the areas to which I've been deployed. As such, innovation ensues, and we've come up with a number of different types of chairs, using said wood and other assorted scrap parts.
The first and simplest is the wooden stool. This simply involves a flat piece of plywood, with a few two-by-fours used to support the sitter's weight. The advantage to this is that it's quick to make in a pinch - for instance, an impromptu spades game. Nobody likes sitting in the sand. The disadvantage, of course, is that it's bloody uncomfortable. Your ass starts to hurt after about ten minutes. An obvious extension, with the same caveats, is the wooden bench. Same idea, just longer with a few more supports. From this simple design, it's fairly easy to add a back, armrests, or what-have-you to this particular design after it's built.
In any field setting, military cots are common. Two of these, if unused, can be put together to make a cot sofa - seating for three, although the middle seat sucks, due to the presence of the support bars in the cots. To make these, you need two fully set-up cots. Simply place the bottom legs of one into the cot resting on the ground, using the built in gaps in the cot's canvas. Fairly sturdy, although sometimes it may need to be shored up in some way - tied together, supported by a convenient building, etc. It seems to depend upon how new the cots are. Once again, these are easy to make, and an additional bonus is that it doesn't require a hammer and nails - ideal tools for making any of the other ones. Drawbacks include the middle seat, already mentioned, and the side bar of the cot used for the backrest - it tends to get uncomfortable if you're sitting up straight. These are best used for quick naps, and should only be sat upon while slouching.
In today's wartime environment, many times, vehicles get blown up, or the seats on them are rendered unservicable for one reason or another. This leads to the next innovation - the Hummer seat! It's pretty simple - a wooden box is made, on which you place the old seat. Extremely comfortable and very sturdy, as long as you secure the seat with a bolt, to prevent the back of the seat from suddenly moving to a reclining position - very uncomfortable at best, embarassing (as you pick yourself up off the floor among the laughs of your fellow Marines) at worst. The major advantage here is comfort. One could fall asleep in one of these seats, not to mention the fact that the bolt can be slipped out, the seat reclined, and the bolt slipped back in. The box extends far enough backward to prevent tipping and falling on one's head. The biggest disadvantage is that it's heavy. Not good if you're sitting at a desk, and need to get up for whatever reason. The chair will easily weigh 50-70 pounds, depending on the quality and thickness of the wood, the type of seat used, etc. Also, on a hot day, one tends to sweat, owing to the thick polyurethane foam used in HMMWV seats.
Yet another concept created by our think tank involves a broken cot and a wooden frame. By nailing the canvas of the cot (bars removed) to a wooden frame arranged (generally speaking) as a hollow recliner, one can make a very comfortable chair. It has all the comfort of a HMMWV seat, but stays cool like a hammock would, since there's airflow through the bottom. There's only two problems. First of all, it's hard to get out of unless built perfectly - limitations in designs so far seem to have required the chair to be built low to the floor. Secondly, due to the size of the cot canvas, this chair cannot be sat in by the bigger among us. It gets pretty cramped for a big guy, given the fact that my shoulders are broad enough that I can't really sleep all the way on a cot in the first place.
The final field-expedient chair I'll cover in my lesson here is the pallet chair. Just take any two standard wooden pallets, remove the bottom two boards from one, and place the "legs" made this way through the second set of holes from the bottom. Mess with it a bit, and you've got yourself a simple, low-riding chair. Pretty comfortable, since it's got a back to it - the problem is that typically, pallets are in short supply. One usually has to acquire them, in sometimes sneaky ways.
In closing, I'd like to place a little disclaimer here - I'm not a grunt. I'm not the one out there, fighting and dying for this war. I'm a communications technician, and as such, I make sure that the links stay up between the sites, so that those grunts that -are- out there doing the fighting have close air support when they need it - and hopefully keep them from dying. We sit long hours, staring at this stuff and praying to whatever supreme beings we believe in, that the stuff doesn't break. That means long hours on one's ass, doing nothing. Trust me, it sucks more than you could know if you haven't been here.