By Matt Godbolt. Based on Tom Walker's b-em emulator. Huge thanks to him for open sourcing his code. Big thanks too to Richard Talbot-Watkins for his help and support. The disc loaded up by default is the amazing Elite (thanks to Ian Bell for making it available).
Source is on GitHub. Works best in Chrome or Firefox. Still very much work in progress: keep coming back for bugfixes and more features!