There used to be a thread on the old forum about BusNinja - a firmware for Arduino (and other AVR boards) allowing to control peripheral components in human-friendly, interactive manner, using BusPirate syntax:
http://arduino.cc/forum/index.php/topic,8464.0.html
I recently discovered this tool and find it very useful and helpful, and did some further hacking on it:
Features which are stable/well tested get submitted to the original project, and so far everything was accepted 
:
So, just wanted to remind of this tool, and hope it will be useful for other folks too.