My absolute favourite waste of time is back with version 4.0. Incredibox is a free DJ/Techno web based application that allows you to be the "DJ"and mix some absolutely incredible beats. You can even record them and stick them on your iPod or other mp3 player.
Sit back, crank up your speakers and dive into the world of INCREDIBOX.
P.S. - IF you have not tried versions 1-3, do them first before going into version 4.0