Piet

Piet on the Net

Mind Candy's Perplex City alternate reality puzzle game featured a Piet puzzle card as part of its £100,000 2005-2006 competition!
Thomas Schoch has some beautiful Piet programs.
Piet was featured in Round Two of the 2002 MIT Mystery Hunt.
Pedro Gimeno Fortea has written an article about esoteric languages (in Spanish), including a discussion of Piet.
Guy L. Steele ("The Great Quux") of Sun Microsystems and Richard P. Gabriel of IBM Research present Piet as part of their "50 in 50" talk about the history of computer languages, which they have presented at several major computer science conferences! You can see the entire presentation here; Piet is introduced around the 20:25 mark.

Piet discussion forums. Note: These are hosted by an interested third party, not me.
Marc Majcher has written a Piet interpreter in Perl, documented here. This version has a buggy interpretation of crossing white blocks.
Sylvain Tinitillier offers a bug-free perl version (zip file here).
Erik Schoenfelder has written npiet, a very nice Piet interpreter with the ability to print program traces and a supporting Piet editor! It's feature-packed and the best version I'm aware of.
Mark Schad has written HIPI, a graphical IDE and interpreter for Piet.
Oscar Rodriguez has written PietDev, a graphical IDE and debugger for Piet, which runs in your browser using JavaScript!
Ross Tucker has written a python version of Marc Majcher's interpreter, Piet.py (text file). Ross notes that he thinks Marc switched the ASCII and literal values for the different input and output commands, so he has swapped them. Ross has also corrected the white block bug.
Steven Anderson has written a Piet IDE/Interpreter/Debugger called Piedit, available here, or here.
Sergei Lewis has written a Piet assembler - it takes instructions and converts them into a graphic program! Available from here.
Casey Link has written Piet Creator, a "robust cross-platform GUI IDE development and debugging tool for Piet, that embeds Erik's fantastic C interpreter npiet". It is available here with C++ source code, and should compile and run on Linux, Mac, and Windows.
Another interpreter, written in C for Linux, by Frank Zago.