My friend used it as the OS for an acoustic levitator built in the US and sold to a Japanese Company - he got sensible hardware control and a GUI toolkit all in the same OS.
I worked for a startup trying to build a phone - I'm NDA on that so can't say much about it except we incorporated Sql lite into it easily enough - the beauty of an OS in C, tack on a 9P interface and you don't need drivers or headers in your Limbo code.
In fact that's the greatest takeaway from Plan9 and Inferno for me. Add 9p to your server and you never need native drivers again - it's a bit like REST. Once you can mount 9P you compose all sorts of stuff.
The canonical example is TCP. If I can mount your /net/tcp on my file system I can use your network stack. Even if I am only connected to you with a serial cable. And it is on a per process basis so I can have two shell windows importing two network stacks from different remote machines and use two networks and they are separate.