I have a Herman Miller Celle at work and love it. They're a bit cheaper than the Aeron, and as far as I can tell offer a couple more adjustments not available on the Aeron. (Lumbar support, seat depth, and seat angle(separate from the back).
There are also some Steelcase Leaps laying around which are also great.
There are also some Steelcase Leaps laying around which are also great.