Brainfuck.

Because it's fun.

My favorite Brainfuck interpreters.

http://brainfuck.tk http://copy.sh/brainfuck/ http://www.iamcal.com/misc/bf_debug/

This is a good IDE for Brainfuck.

http://4mhz.de/bfdev.html

If you are interested, you can contribute to this Brainfuck compiler optimization project.

http://www.nayuki.io/page/optimizing-brainfuck-compiler