Not sure I'd call it a successor as much as a cousin. Inferno is certainly based upon Plan9, but it is not Plan9. If you want a more up to date or maintained Plan9 check out 9front[1] or 9atom[2].
I've only ever toyed with it but Inferno has some interesting properties. They've got a system that will run the dis virtual machine on a 32-bit cpu without an mmu.
I remember a couple of Lucent telecom products from the mid nineties that ran on it but I'm not sure what else was out there.
[1]: http://9front.org/ [2]: http://www.9atom.org/