Veerle has a great step-by-step tutorial on how to create really nice, readable pie charts in Illustrator CS2.

There are plenty of chart making applications out there, but I always liked Illustrator's simple yet useful capabilities in this area because it allows me much more graphic customizing capabilities than even Excel. Just keep in mind that charts, by their very nature, are difficult to read. Keep them simple, clean and use contrasting colors to differentiate data areas.