The brainfuck language is an esoteric programming language noted for its extreme minimalism. It was designed to challenge and amuse programmers, and is not suitable for practical use. Its name has been variously bowdlerized, as in brainf*ck or brainfsck. The name of the language is generally not capitalized, although it is a proper noun.
More about brainfuck on esolang.

Ook! is a joke esoteric programming language created by David Morgan-Mar. It is identical to Brainfuck, except that the instructions are changed into Orangutan words.
More at esolang.

Spoon is a brainfuck-like language. Its operators consist of 0 and 1.
Spoon homepage.

HQ9+ was designed by Cliff L. Biffle and it has just 4 commands(H, Q, 9, +). HQ9+ homepage.

Whitespace is THE language if you want to hide something from other's eyes. If you print a whitespace program out, you'll get a completely blank page! Nobody (even you) will be able to get the information back which you encoded in the program.
Well, seriously, Whitespace is a language where only whitespace is used to code a program, in contrast to nearly all other languages where whitespace is completely ignored. As Whitespace ignores all other characters which are non-space, you can easily integrate a whitespace program in for example a c program.
For more information to that language see the Official Whitespace site.

Byter is a language for training your brain. It consist from 11 instructions that are intended to move pointer and outputing characters.
More about Byter.

Befunge-93 has an instruction pointer which cannot move only in one dimension, that means, forward and backward, but it can travel in two directions: Up, down, left and right. Instead of having a special syntax for comments for example, you can simply direct the program flow around the parts where your comments are. For more see the language specification on Cat's eye .