I have a small tool bag with a 3lb short handled hammer, a set of mm/sae wrenches (no duplicates. example: 9/16, 15mm, 5/8, 17mm, 18mm, 3/4 etc. ) stubby and standard screwdrivers, one of those multi-bit drivers for all the misc torx (hint: you can use a 1/4" wrench or socket on the bits), a couple vice grips, combo wire cutter/stripper/crimper, channel locks, needle nose, pipe wrench, duct tape, electrical tape, misc wire connectors and terminals, a few feet of #14 wire, a small roll of thin baling wire, a trailer wire connector schematic, a small multimeter, some zip ties, and a few rags. There may be a few more tools in that bag, but that's the big ones. It covers most situations and fits under the driver's seat. I have all the factory tire changing equipment and it will still work on my truck. I carry a 60" high lift jack and the handle comes off easily and makes a great cheater pipe. I also have a small 4 way zip tied to the top of the spare tire on my trailer. I have a nice tool set in a plastic case that I have added additional tools to that can be thrown in the toolbox anytime I think I might need a few tools. I keep a roll of those blue shop towels under the back seat. They're handy for emergency toilet paper as well as being rags. Right next to them is a couple pairs of gloves, a fire extinguisher, assorted ratched straps and bungee cords, and my trailer wire adapters. I keep bottled water in the door pockets.
Also, I carry some basic recovery gear. I leave my winch at home most of the time, but I always carry a 30' snatch strap, a receiver shackle bracket, a couple extra shackles, and a couple of those reciever hitch pins with the quick release on the end. That way I can use the strap on someone else's reciever hitch without having to rely on them to be prepared. I also keep a 10' choker strap that can be used to spread the load to both towhooks or as a tree-saver. I have a 8' piece of 3/8" chain that I grab whenever I think I might need it.
ALSO: Jumper cables. You wouldn't believe how many people don't have any of these. While good ones can be a little pricey, you can get a very basic set for 6-9 dollars that will get you out of a pinch.
I'm sure there's more that I'll think of later.