Starting the search, any multi-day adventure races out there and that aren't just running or crazy Anvils? Hike/Run/Water/Bike/Hitchhike...

What races do you know about? What adventure races have you seen or participated in?

A few running friends are looking for an adventure and have asked me to tag along since they think I can handle a multi-day type 2 slogfest of a race.

Thanks for any input.