Lords of the Earth. Buddy of mine, Tom Harlan, started it when we were in high school, it was the first Play-by-mail - we used paper! - that I got really sucked into. And epic...Almost twenty years later, the same game is going strong, still run by Tom...and there are lots of other campaigns....see:
http://www.throneworld.com/lords/index.jsp
Once that got me, it was all downhill, and here I am...