Official Sites:

Play-Doh
Hasbro's official site for their Play-Doh product line.

Elsewhere on the Net:

Clay-O-Rama
Clay-O-Rama is a combat game played with Play-Doh monsters you make yourself.

Mr. Bill
The official site for Mr. Bill, a Saturday Night Live character made from Play-Doh.

Recipe Using Food Coloring
Make your own Play-Doh at home, using food coloring.

Recipes for Play-Doh
Three recipes to make your own at home, using Kool-Aid or peanut butter.

Recipes for Play-Doh
Recipes for several different craft compounds, including Play-Doh and Silly Putty. At last count, there were seven "playdough" recipes listed.

The Unofficial Play-Doh Site
A history of Play-Doh, including advertisements from years past and a look at the "Play-Doh Boy" as he has changed since the 1950s. Most interesting of all is the archive of Play-Doh products, such as the Jabba the Hutt playset.

Yesterdayland Toys
An essay (with timeline) about the history of Play-Doh.