The Jelly-Brain Jinx (incantation unknown) is a spell that presumably affects the target's mental processes. During the September 1999 riot that took place in the middle of the Puddlemere United vs. Holyhead Harpies Quidditch game, many of the Harpy supporters were using this jinx.
This spell may also have been the spell that the Death Eaters hit Ron with during the Battle of the Department of Mysteries.