Cute RPG where limited parts of the level code itself can be overwritten to solve puzzles (e.g., in one level, changing the behaviour of a door such that opening it does not remove the key from the player's inventory).
Fun game of practical XSS. Some amazing tricks there and I am confident I have left some of these holes open in the past.
Open a small floating pane with common accented and special characters.
Incredible: a significant subset of Firebug, also compatible with IE and Opera, in a bookmarklet.