Whitespace
That new Whitespace programming language looks fun. It uses only three different whitespace characters. So I've been thinking about a possible language with just a single character. The only information contained in the source code of such a program would be the code's string length. The compiler would have to read all instructions from the properties of this number, e.g. its digits, its prime factors, etc. I couldn't come up with anything that looks even remotely feasible though. (The cheap trick of course is to interpret the string length as the Gödel number of some C code.)