Apart from the LEGO brick, is there any other hardware that has an NXT firmware written for it?
LEGO itself does not make specific firmware/software to support other devices other than the official NXT brick but...

...you can still use Arduino and Raspberry Pi (and more) with the NXT components including the NXT brick.  Personally, I mix LEGO and electronics all the time.

Arduino &amp; Raspberry Pi are flexible development platforms and they can easily be interfaced with other devices. Also, the NXT devices (i.e. The Brick or peripherals) can be interfaced by something else than the official LEGO hardware &amp; software.
On the other hand, the effort needed to run the LEGO firmware (probably through an emulator...) on a new device is way harder than to interface the systems.  Remember that this is a firmware and it expect to run on the exact device it was built for.

Arduino &amp; NXT:
There is a lot of Arduino-NXT hardware components that you can buy or even make yourself.  For example you could use this Arduino shield called NXShield-D and it's software.

RaspberryPi &amp; NXT:
Pi is still relatively new but, since it's a full computer, it is very easy to use.  To interface with NXT you can use the GPIO pins but thanks to the USB port, somebody already made the NXT-python to control the NXT brick.