The video is well worth a watch, but for those short on time, Mirage OS allows you to build networking apps in OCaml that run (either on Xen or *nix) with their own integrated kernel and customisable network/storage/processing stack.

Why would you want this? High performance, less resource overhead, better security (whole networking stack is type checked). This (and similar projects to it) could represent the next generation of web platforms.