Feel free to add your own application or site to the list
Front Row Education. Every web request is served by Haskell. The application students learn Math with has a large volume of users and a lot of requests. Yesod doesn't use much CPU. The DB is the bottleneck.
DoxIQ: Document viewing analytics. Every document view creates multiple requests to Yesod for analytics. When a document is popular and seeing thousands of views in a short time frame, Yesod keep operating smoothly.
Yesod applications that you can run locally.
The following sites are powered by Yesod. Feel free to add your own to the list: