Authors and Illustrators
These two are by far the best collections of author websites. Check here first:
Children's Authors & Illustrators on the Web
If you didn't find the author you were looking for in the first two websites, then check out these sites or use a search engine to find the author:
teachers@random - Authors/Illustrators Biographies
SimonSays.com Author Information
Children's Authors & Illustrators on the Web - Other Sources