I'd been meaning to play with Racket's built-in sandboxing capabilities for a while so yesterday I sat down and made Try Racket. It's a web app that lets you type in Racket code and run it. The code you run is tied to your session and each session is allocated up to 60 seconds of run time per evaluation, with up to 128MB of memory used. Filesystem and network access is not permitted and neither is access to the FFI. The application itself runs inside a further-restricted Docker container.
You can find the source code on GitHub. Contributions and improvements are totally welcome!