Yep, I've heard of Tough Mudder. I learned about it around the same time as I was doing Spartan races. I got immediately turned off from Tough Mudders though, when a friend did one and got electrocuted on his forehead and was knocked out for a few minutes. He came back to and finished the race (he's insane), but that was a major turn off!
Do Tough Mudders still do the dangling live electrical wires?
Also, on a more general note... To me, Tough Mudders seemed to be more about shocking your physical system and seeing how much it could take, whereas Spartan races seemed to be more about physical strength and endurance in terms of strength. The latter appealed to me much more!