http://www.recoverygear.com/bags.html
I thought that this was a pretty good kit, You could build a kit yourself with this stuff also. I am afraid that all my off-road experience comes from driving trucks in the Marines. We carried a variety of things with us to help us, most involved some form of physical labor (20 marines on a rope to right a Hummer, all the kings horses and all the kings men to right a 5 ton when there was no chance of mechanized assist)
Some good items have been mentioned so far, but the basics are always important to remember.
1. You need to have several sets of good leather gloves.
2. You need to have several pieces of strong synthetic rope. (say 4 50 foot sections)
3. You will want to have D-Links rather than Beeners because they are typically stronger, and once closed...you know they are closed.
4. Depending on terrain you might want to have several different ways to tie off. There won't always be trees, and there won't always be rocks. You might need to find a soft soil anchoring method to provide the leverage you need. I found that using a screw-style anchor in the marshy-sandy soil of North Carolina helped. YMMV.
5. Spare set of clothing for recovery. I have been on some recoveries that claimed my pants...oh yes...split right down the crotch. You don't feel so macho when your diddles are dangling out.
6. Safety goggles, and possibly head protection. You'd be suprised what can happen when 3 tons decides to go your way.
7. two shovels, 1 pick, and a sledge hammer. Digging, digging, and placing alternate anchor points.
8. Water. Depending on where you are and how long you plan to be there.
9. Enough flashlights and batteries to see what you are doing should it be late.
10. Find someplace on your rig to mount a pry-bar that is about 6 feet long and made of a material that won't break when there are 6 people pressing on it.
That does it for what I can think of right now. All these items were useful when righting hummers or our rovers (special forces use rovers). I will admit though, we didn't have access to a lot of the fancier recovery tools. So this is more geared towards people power than Snatching or Winching.