Unfortunately, I'm not sure there is one.
The problem is, as far as I can tell, there aren't any, or at least there weren't three years ago when I last looked. None of the knockoffs have the combination of support and breathability that makes the Aeron unusual.
One thing I'll say: if you're looking for Aerons, make sure you get one with lumbar support, which makes a tremendous difference. I've heard good things about Steelcase chairs -- see http://www.steelcase.com -- but they're not cheap either. We talk about them briefly here: http://blog.seliger.com/2008/06/15/tools-of-the-trade—what-a... .