BrainFuck
The BrainFuck programming language (originally by Urban Mueller) is beautifully minimalistic, elegant, very difficult to use and, of course, Turing complete. The language gives you a finite tape that can hold natural numbers, and a pointer. There are only eight commands:
| > | Moves the pointer one step right |
| < | Moves the pointer one step left |
| + | Increments the current cell |
| - | Decrements the current cell |
| [ | If the current cell is zero, execution jumps to the matching "]" |
| ] | If the current cell is non-zero, execution jumps to the matching "[" |
| . | Prints the ASCII value of the current cell |
| , | Inputs a character and fills the current cell with its ASCII value |
I have included two extra commands based on the ones suggested by Blake Rhodes for numeric I/O.
| : | Prints the integer value of the current cell |
| ; | Inputs an integer and stores it in the current cell |
Due to the commands, BrainFuck programs have a very simple flow structure, having no "goto" (or "come from") commands. Before you get worried however, rest assured that it is still practically impossible to understand a BF program from the source code.
The first thing you may think about this language is "How am I supposed to do anything in it?". The answer, you will be relieved to know, is "With great difficulty.". Don't despair, though, that is the whole point of the language.