Here is an interpreter for the Brainfuck programming language.
It's ok if you aren't in the mood for writing some Brainfuck, I understand. I've linked the Brainfuck source code to the hello world program and the rot13 program here, so that you can simply copy and paste the source code into the interpreter and execute it here.
I've also written a Brainfuck program generator. The generator generates the source code to a Brainfuck program that outputs a string of your choice. So instead of just "Hello World!", you get to choose what your Brainfuck program prints out.