Submit portable freeware that you find here. It helps if you include information like description, extraction instruction, Unicode support, whether it writes to the registry, and so on.
The Powder Toy is a physics sandbox game, which simulates air pressure and velocity, heat, gravity and a countless number of interactions between different substances! The game provides you with various building materials, liquids, gases and electronic components which can be used to construct complex machines, guns, bombs, realistic terrains and almost anything else. You can then mine them and watch cool explosions, add intricate wirings, play with little stickmen or operate your machine. You can browse and play thousands of different saves made by the community or upload your own – we welcome your creations!
I messed around with this for a really long time without even noticing the time. Fortunately someone interrupted me or I would have played this well past the 2.5 hours I spent on it.
Apart from SYSTEM's chess game, there haven't been many games on the site in a while and given the uniqueness of Powder Toy, it seemed like a good way to balance that.
Also I think it's important to highlight great GPL games.
Note that I'm not totally clear on the OS support level but while Softpedia lists it as Vista or above, I tested it on WinXP without issue. I imagine it works on earlier windows versions as well but I have no way to test.