I'll soon be returning to school, and living in an apartment. I figure I need a good programming chair, and there are have been a few discussions about them, from Jeff and Joel. But what about for a small space and on a small budget? What's the best chair I can get that (1) doesn't take up a lot of space and (2) doesn't cost a lot of money?
I have tried the Space Air Grid 5560. It is a smaller chair still mesh back. It doesn't have the features of the HM Aeron but it is $199 and provides good support for your back.
Sit4Less link is below.
http://www.sit4less.com/products/office-star/space-air-grid-5560-black-/127
I use a yoga ball at home and at work, very inexpensive (~$20 or less), comfortable for long periods of time, and fairly compact.
I suggest trying your nearest office furniture liquidator. These are stores whose sole purpose is to move new/used office furniture.
I picked up this Topstar New Artwork for $130. It was a demo floor model in it's previous life, and now it is mine.
Although you say space is an issue, I use this chair in my small apartment, and by small I mean 12' x 15'. It's where I spend most of my time anyway, and if I really need to free up some space I slide it under my desk.
If space is a real issue, there are a lot of different companies that are making stool-like office chairs like this. Jeff Atwood mentions one of these called the Swopper in his Coding Horror blog post Investing in a Quality Programming Chair.
Good luck.