Wrenches
A good set of ratcheting wrenches, combined with a small assortment of adjustables, should cover any size you might need. One thing to note before you leave is what type of truck you are looking to pull parts from. This will determine if you need to bring either the metric or standard increments.
Screwdrivers
Pick up a universal-style screwdriver and an assortment of tips. This should cover any screw you might come across, even the smaller Torque heads. Then, all you need is one flat tip and Phillips just to be on the safe side, and if you have room, throw in a large prybar.
Electric
Not many yards have available power, so if you take an electric impact, screw gun, or reciprocating saw, just make sure you charge the battery before you leave. There is nothing worse than grabbing the screw gun, pulling the trigger, and having it go click. Not only do you have to pull the screws manually, now, you still have to lug the thing around. Talk about dead weight.
Engine-Checking Tools
If everything looks good in the filter (see below, right), then you might want to do some of the basic tests to find out if all is well. You will need a dual leak-down tester to check the condition of the rings and valves. A starter button will help you turn over the motor to see if it will even move and if there is any vacuum being produced. ProForm's self-powered timing light works great, because it doesn't require a battery in the vehicle to run it and it doubles as a flashlight.
Battery Power
All of the engine-checking procedures (mentioned later) will necessitate you turning the motor over, and most yards pull the batteries when the vehicles come in. Booster boxes, like this one from Peak, will allow you to crank the motor over and run your checks. This will also help you check other electrical components like power windows and power seat tracks.
Sockets
The 1/4-inch drive sets work great for the yard, and they don't take much room. Combine that with just the bare essential in the larger drive and you should be fine. Remember to bring an assortment of extensions and at least one wobble socket for those off-angle nuts.
Oil Filter Cutter
If you are looking to buy a motor for the yard, you'll want to do some pre-checks just to make sure it's worth the purchase. An oil filter cutter will give you some insight to the condition of the internals. If there are a lot of metal shavings in the filter, you might want to move on to a different vehicle.
Mobile Air
Here is our favorite setup for getting sheetmetal pieces. The Powertank CO2 bottle holds 3,000 psi of the non-flammable gas, which will run the saw long enough to cut any filler piece you might need. Not only that, if you don't have an electric impact, this will run your air impact for those real stubborn bolts.
Allen Keys
There are many options for Allen keys- straight, bent, and assembled sets-but whatever you choose, just bring one. We prefer to take the assembled set with us to the yard, and it has never let us down yet.
Miscellaneous
Make sure to bring a small can of penetrating fluid for those rusted bolts and a small flashlight just in case you have to look under a dash.