This is a web-based interpreter for the esoteric programming language Pail.
Technically, it is
Pail.lhs (the reference implementation
by the Haste compiler, running in an HTML5 document.
Select an example program from the dropdown box and click "Run" to run it. You can also edit the program before running it. Or you could clear the textbox and write an entirely new program there and run it.
You might also want to read the description of the Pail programming language.