Socket.IO powered all of the real-time editing capabilities - high performance, easy-to-use, and awesome.
We used MongoDB to implement a filesystem abstraction for the application (i.e. each user has his/her own home directory and can share them, etc.)
Blueprint has awesome SPA components and a fantastic icon set that you can use to build a very rich UI for code authoring applications.
We used Redis to implement a locking mechanism for ensuring consistency while multiple users were editing the same file.
Used extensively for all front end development work - excellent native TypeScript support and tons of plugins.