The Prisoner's keyboard
The premise of the Prisoner's Constraint is as follows. A prisoner, having secured a pencil and a small scrap of paper, wants to cram in as much text as possible. So he avoids words with ascenders (bdhfjklt) and descenders (jpgpqy).
The Prisoner's Keyboard is a keyboard extension that lets composers create this and other constraints by removing letters from the mobile keyboard. It is available for iPhone and iPad here.
Once forbidden letters are removed, the composer regains the freedom to generate permissible text with the keyboard.