I did one a while back where I got a spigot that had male pipe threads on it, and then just drilled a hole in the barrel a hair smaller than the threads. The spigot threaded right into the relatively soft plastic of the barrel, and never leaked a bit.

That said, there are spigots made specifically for the task:
http://www.amazon.com/Rain-Reserve-2.../dp/B004RYF410