One of our great readers Ivan just sent this through to me.
He has created a video explaining how a PID (proportional / integral / derivative ) controller works and shows an example of it running on a simple NXT robot towards the end.
Now the video is in Portugese (who says we don't cater for the international audience! :D) and although I've only done a semester of learning Spanish at university, I was able to keep track of what was going on.
Even if I didn't understand any of the writing (he goes through a *lot* of post-it notes!), it was still great fun to watch, the style, pace and music of it just perfect. I loved the little things that happen throughout, such as running out of room to write and having to add another post-it, and trying (unsuccessfully) to attach the NXT with masking tape :)
Ivan's offering to put in English subtitles, so if you're interested, please make a comment!
**EDIT: Ivan has added English subtitles! :) **