When I hit U-Pull-It lots locally (or at least when I'm driving there...), I tend to take a fairly stupid number and variety of tools along with me- easily well more than I use, but at the same time I've rarely found myself cursing not having the tool I need to pull something.
However, the next trip I'll be taking and hopefully doing some salvage-yard hunting will be one where I will have to fly- so I need to significantly pare down what I plan to bring. I'll be checking baggage so I don't have to limit it to what can be carried on the plane- but at the same time, I have to be somewhat conservative as far as overall weight.
So I'm looking for suggestions as to what you'd consider 'essential' tools for a travel U-Pull-It bag, info on what you have in your (stories more than welcome 
), as well as fancy tool suggestions to cut down on the overall volume/mass I'm carrying along.
I stick to imports, so I just need metric stuff. Realistically, with a 10mm, 12mm and 14mm socket and box (Crescent) wrench, along with a medium sized phillips and flat screwdriver, a Channel Lock and a can of BP Blaster I can get 80% of the stuff I need. I never go without a Leatherman Wave of course.
I usually pack a set of GearWrenches with adjustable heads, a small 1/2" drive metric impact socket set with two extensions, a breaker bar, linesman pliers and a flashlight. With that I can get up to 90% of what I need. (You can usually find pry bars, lug nut wrenches, and hammer substitues in the yard).
For the last 10%, you just have to bring the big tool box. You run into crazy stuff out there and you never know what you need. Just walk back to the car and grab the Toyota-Specific o2 wrench or GM panel removal tool or whatever.
I have one of these:
http://www.amazon.com/Tool-125-1010-Switch-A-Bit-Multi-Bit-Screwdriver/dp/B000BDH9M8
It is damned handy. I think I need to pick up a couple more to put in the travel tool kits.
For travel and salvage yard work, a good set of tools that can be used on either metric, SAE, damaged or just wonky fastners can help:
http://www.amazon.com/Metrinch-Combo-Spanner-Socket-Wrench/dp/B000WR3JUO/ref=sr_1_1?s=hi&ie=UTF8&qid=1351786354&sr=1-1&keywords=metrinch
A little expensive, but there are similar sets of tools around for less money. The biggest thing I find I need are either a set of ratcheting box wrenches and/or a breaker bar with a 1/2" socket set. Those last two items tend to take up a lot of room and add a bunch of weight.
I'm also metric-only.
I bring:
8, 10, 11, 12 sockets in 1/4" drive.
10, 11, 12, 13, 14, 17 in 3/8" drive.
12, 13, 14, 17, 19, 21 in 1/2" drive deep
1/4, 3/8, and 1/2" drive ratchets, each with 3 and 6" extensions.
8, 10, 11, 12, 13, 14, 17, 19, 21 wrenches.
One adjustable wrench
One pair of vice grips
One pair of channel locks
One pair of needlenose
One pair of wire cutters/side cuts
One hose cutter
One breaker bar
One razor/box cutter/knife
Two each phillips and flathead screwdrivers one small one large.
It's rare i get stumped. 
I'm mainly going to be going after GM stuff (parts for the El Camino project), but I've generally been more metric-focused... thankfully though most of the time that works out well- I think that it takes more metric tools than SAE to pull a set of heads off a Vortec engine, for example.
If I get bored, I'll document what I have with me in my rental car for this 2 week business trip... it's pretty ridiculous. :P
That Metrinch set looks rather nice, though as pointed out, expensive. I've also actually found recently that the spline-drive sets that now are my primary 3/8" sockets do NOT work on some fasteners even though they're supposed to- the most prominent example of this is that center-bolt valve covers on a SBC are a 10mm bolt, but the spline socket 10mm will NOT grab it enough to break it loose- I have to dig out the 6-sided 10mm from my small Gearwrench set (which will be coming along, it's quite nice, has a ton of Torx bits, and is compact) to break them free.
