emeeks’s blocks by Mike Bostock.
From the about page:
This is a simple viewer for code examples hosted on GitHub Gist. Code up an example using Gist, and then point people here to view the example and the source code, live!
The main source code for your example should be named index.html. You can also include a README.md using Markdown, and a thumbnail.png for preview. The index.html can use relative links to other files in your Gist; you can also use absolute links to shared files, such as D3, jQuery and Leaflet.
Rather remarkable page that includes a large number of examples from D3.js in Action.