I just participated in one of these today. It was without a doubt one of the hardest events (they don't call it competition) I've been a part of. 11 miles. On a ski mountain (up and down and up and down and up and down). With 17 obstacles along the way, including; scaling walls, climbing rope ladders, running through fire, carrying logs, monkey bars, mud pits, ice cold lake swimming, and lots more.
It's really an amazing event. They stress teamwork and camaraderie the whole way, and at each obstacle people help out their fellow tough mudders. At least one of the obstacles is simply not possible on your own.
5000 people participated today, and another 5000 are doing it tomorrow (here in PA). And the event travels around the country, so their most likely one close to you at some point during the year. If you like to run, or push yourself to extremes, this might be up your alley. As tough as it was, it was a really great experience. Also, there are prizes for best outfit, best mohawk and mullet (free shavings afterwards), and even free tough mudder tattoos to for anyone that wants them (I opted out). And if you think the event isn't tough enough, they'll give you 20lb weights to run with! The events raise money for wounded war vets.