Not a wrench, but I bought a short handled hammer (basically a grip and a conventional head) It fits in the underseat toolbox. I use it to tap pins in or out when changing 3pt implements. Got it at Harbor Freight for about $6 if I remember. Mine is ball-peen but they come in claw as well. http://www.harborfreight.com/cpi/ctaf/displayitem.taf?Itemnumber=95930
I also keep a pair of pump pliers and a cheap screwdriver on the tractor.