Well, it depends on the edition of Epic you are playing. If it's Epic Armageddon (the most recent), the latest and greatest is downloadable from here:
http://www.net-armageddon.org/page/rules
It's a fan site that basically put out a new, better edition of the Epic Armageddon rulebook, without all the errors that were in the hardcover, and they even added in the Eratta and
FAQ's on each page they pertain to.
Also, there are tons of army lists in the back. They are either tweaks of the original Armageddon army lists (Space marines, Orks, and Eldar) or new fan-made lists that have actually gone through quite thorough playtesting on the Tactical Command Forums (so you get things like Chaos, Necrons, Tau, Speed Freak and Feral Orks, and several variants of Space marines. So it's basically the official
GW rules plus much more fun stuff.
I never got around to playing the older versions of Epic when they were new, and this is the ruleset that I am using to enter into the game and teach my friends.
There are however several other Editions of Epic that play very differently from this one-some were more complicated, some were more simplified. Epic Armageddon is (in my opinion) the best mix of the two ends of the spectrum.