There is nothing sweeter than settling into an ergonomic office chair. You've tried the fancy ones in the store, but how many of us are really going to shell out $600 for a top of the line Aeron. Jon Aquino is certainly not. He's hacked together a wonderfully geeky and apparently comfortable chair for his home office. He used things you'd have around the house anyway, making the cost virtually nothing.
All you need is a regular chair, a telephone book, a cutting board, a towel, and a bungee cord (or belt). Check out Jon's post here for assembly instructions. It's really simple and the height along with the lower back support are adjustable. Just look at his posture in this picture. He's saving himself a lot of back pain and probably getting more work done as a result.
Some commenters wonder if sitting on a phone book would be comfortable. I imagine you could put something over top or use a firm cushion instead.