Here's a sample Haskell & Yesod application demonstrating a basic REST API. It could serve as a starting point of a toy project: Haskell & Yesod - REST API.
I wrote this application as a supplement to the "RESTful Content" chapter of the "Haskell and Yesod" book. I felt the examples given in that chapter, while good, could be further developed into a working application (with IO, custom types, etc.).
Your feedback is welcome and please expect some changes to that application as time goes on.