I've been looking into building one and found that many masonry yards are selling kits - like this:
But hands-down, the coolest fire pit I've ever seen - and it's a very "green" idea - is making one out of an old washing machine drum - no joke!
There's a video on it here:
http://www.youtube.com/watch?v=If7qSBvt_Ng
But the stone work is a little crude - I'd build it up a little nicer than that, myself... maybe find some old discarded brick for character...