There are lots of sources for coop design out there, depending on how handy you are with a hammer and nails and whether you need explicit step-by-step instructions or simply a book of inspirational pictures you can create your own plans from.
- The Garden Coop -- Step-by-step detailed plans available for download. John Carr's put together a really useful design and done a great job making it accessible to those of us (like me) who have a lot of enthusiasm but not a lot of practice building structures.
- BackyardChickens.com Coop Design Gallery -- An online gallery that displays the myriad types of coops owned by the community of backyard/urban chicken owners. The individual coops you'll see range from small to huge, from plain to fancy and from cheap to expensive. The amount of detail behind each coop varies. Some have dimensions and as-we-built-it photo documentation. Others are just the finished product.
- Chicken Coops: 45 Building Plans for Housing Your Flock -- a book by Judy Pangman. The title's a bit misleading, but it's still chock full of ideas for coops for your chickens. Use this book more as inspiration for building your own coop, not for plans with a step-by-step guide to building a coop (like the Garden Coop's plans).