The original VP Laboratories demo, released in 2011. Does not work well with the latest version of the game.
Important information! After new Portal 2 DLC the mod menu is broken! To fix this rename "mainmenu.res" in "pak01_dir.vpk\resource\ui\basemodui" to "mainmenu_new.res"! Otherwise, if you dont know how to do that, just select "Extras" in the broken mod menu.