In general a JavaScript-based Linux virtual machine should allow you to run software that would otherwise be difficult or impossible to run in the browser (which right now can be done for a lot of software by compiling it to JavaScript with https://github.com/kripken/emscripten). Data could be transferred to the VM for processing and then back to "normal" JavaScript using serial I/O. This might prove a useful kludge.
For a simple example of where it could be used consider a webmail provider who wants to get GPG encryption into their web UI. The provider could embed a Linux VM in the HTML document to let the user do encryption with the well-examined native version of GPG. (Although native GPG running inside this VM would not be safe from, e.g., malicious JS in banner ads.)
Right now the emulation is too slow to do these kinds of things practically, however, and it's hard to say whether or not it will get sufficiently fast before other tools for running native software the browser fully mature.
tptacek tells me that secure JS is never going to happen so I don't see a use case there. Yes, proof of concept is cool, until someone steals your private key. No thanks.
I suppose though you could non-secure stuff, like add a Python interpreter or C compiler and teach people to code inside the VM. GitHub could provide in-browser syntax checking using a real interpreter/compiler.
For a simple example of where it could be used consider a webmail provider who wants to get GPG encryption into their web UI. The provider could embed a Linux VM in the HTML document to let the user do encryption with the well-examined native version of GPG. (Although native GPG running inside this VM would not be safe from, e.g., malicious JS in banner ads.)
Right now the emulation is too slow to do these kinds of things practically, however, and it's hard to say whether or not it will get sufficiently fast before other tools for running native software the browser fully mature.