Lightness change
Hue change	None	1 Darker	2 Darker
None		push	pop
1 Step	add	subtract	multiply
2 Steps	divide	mod	not
3 Steps	greater	pointer	switch
4 Steps	duplicate	roll	in(number)
5 Steps	in(char)	out(number)	out(char)

The Piet language interpreter begins executing a program in the colour block which includes the upper left codel of the program. The interpreter maintains a Direction Pointer (DP), initially pointing to the right. The DP may point either right, left, down or up. The interpreter also maintains a Codel Chooser (CC), initially pointing left. The CC may point either left or right. The directions of the DP and CC will often change during program execution. As it executes the program, the interpreter traverses the colour blocks of the program under the following rules: [...]

Hello World

Print "Piet"

Primes

π

Brainfuck interpreter

Conway's Life

The Esoteric programming language page is remarkably complete -- I didn't know about a third of these!

I can't even decide if this counts as steganography. Imagine the computational load that could be lurking, un-executed, in museums around the world!