I have a couple Herman Miller 'Equa' chairs, one here at my office and one at home in front of the home computer, and for me they're the most comfortable chairs I've ever tried. They are expensive new, but I picked these up used at an office supply place for a much more reasonable price (there are always startup companies who spend a mint on fancy office furniture and then go belly up a year later, so good office furniture is pretty easy to find.)
Herman Miller has a newer version called an 'Aeron' that has a mesh back and seat instead of padded upholstery; they're nice as well but I prefer the one I have.
