I don't have one, never thought I would need one, but when I broke a rear control mount in Sandstone Cyn farting around on some rocks I sure wish I'd had one.
ORW carries a sweet wire-feed on-board welder, it's about $800 but it's a complete hand held unit, very cool. I'd get one if the budget allowed:
http://www.offroadwarehouse.com/products/productID/3368A popular one I've seen around a lot is the Premier (
http://www.premierpowerwelder.com/in_use.html ), does an awesome job, but requires the welder and an alternator upgrade.
Lots of homemade ones like this:
http://www.pirate4x4.com/tech/on-boardwelder/Not one of those thing that's normally carried, but like I said, nice when it's needed
There's the break (and let me tell you, it's interesting to drive a Jeep with an upper control arm flopping around, time for a long-arm...):