And the number one best invention from a jail cell is ...

The toothbrush.

William Addis of England is credited with creating the first mass-produced toothbrush in 1780. In 1770 he had been placed in jail for causing a riot. While in prison, he decided that the method for teeth brushing of the time – rubbing a rag on one's teeth with soot and salt – could be improved. So he took a small animal bone, drilled small holes in it, obtained some bristles from a guard, tied them in tufts, then passed the bristles through the holes on the bone and glued them. He soon became very wealthy. He died in the year 1808 and left the business to his eldest son, William II.

From Wikipedia