Hello World in Piet with the matching pixel art by Kelly Boothby.
There are also attempts to write a piet program and a piet like art work.
One last great example for a piet program which I like a lot is by Richard Mitton. This program calculates an approximation of pi by dividing a circular area by the radius twice. A more accurate value can be obtained by using a bigger program and therewith circle :)
The objective of allRGB is simple: To create images with one pixel for every RGB color (16,777,216); not one color missing, and not one color twice.