On-the-fly physics simulations seem to be hurtling towards Next Big Thingland at the moment, with Crayon Physics Deluxe having recently won the Grand Prize at the Independent Games Festival, beating other serious contenders such as Audiosurf and World of Goo. You can't actually play Crayon Physics Deluxe at the moment, but you can grab yourself a free beta copy of Phun, a '2D physics sandbox' that will stop you doing anything even remotely productive all day. Draw objects, link them together with springs and chains, drop large amounts of water on everything and generally cause a dreadful (virtual) mess. It works wonderfully - if you've ever fancied building your own Rube Goldberg device, your life is now complete.