Brainfuck

The brainfuck language is a programming language created by Urban Müller, who wanted to create the smallest possible compiler. Hence, he created a language that is made up of pointers. Anyone who knows c/c++ hire is the equivalents of a c++ code:

>	becomes	++p;
<	becomes	--p;
+	becomes	++*p;
-	becomes	--*p;
.	becomes	putchar(*p);
,	becomes	*p = getchar();
[	becomes	while (*p) {
]	becomes	}

This is what a hello world program looks like in Brainfuck:

++++++++++
[>+++++++>++++++++++>+++>+<<<<- array="" br="" in="" initial="" loop="" set="" the="" to="" up="" useful="" values="">>++.                             Print 'H'
>+.                              Print 'e'
+++++++.                         Print 'l'
.                                Print 'l'
+++.                             Print 'o'
>++.                             Print ' '
<<+++++++++++++++.               Print 'W'
>.                               Print 'o'
+++.                             Print 'r'
------.                          Print 'l'
--------.                        Print 'd'
>+.                              Print '!'
>.                               Print newline 

So writing a bigger program is not an easy job to do. Anyway, it's a cool language because as the program gets bigger, there are small chances that anyone other would understand it.
There is an online interpreter made in PHP.