I like to listen to Ted talks while I'm working. Recently I happened across a talk by Niels Different where he talks about his design background and then into his decisions on the Humanscale Freedom Chair. Its one of the major and decently known Task Chairs available.
I tried a Freedom chair out for about a week (not long enough- I had to give it back). Its a really nice chair. Solid construction and a head rest like no other. The first thing you will notice is the firm back that you can still lean back with a bit of force. Once you start leaning back the head rest moves to counter that. Its very different.