A RESTful web framework with strong compile-time guarantees of correctness. It also affords space efficient code, highly concurrent loads, and portability to many deployment backends (via the wai package), from CGI to stand-alone serving.
Yesod also focuses on developer productivity. Yesod integrates well with tools for all your basic web development (wai, persistent, and shakespeare/hamlet)
The Yesod documentation site <http://www.yesodweb.com/> has much more information, including on the supporting packages mentioned above.