Here are most of my favourite sites and games that I've come across. Click the links to go them:
This is my favourite game. It's a free, downloadable, desktop simulator that you can use experiment with different elements. It has endless possibilities, and you can build real working models of anything from a nuclear power plant to a computer. You can upload your simulations to their "cloud" to share with others, and download from another computer.
This is very similar to Powder Toy, but this one is in the browser. You can play it anywhere, as long as you have Java. Powder Game was the original idea for Powder Toy, and is quite old now. It also is less advanced and has a smaller simulation area because it must be run in a browser.
Another one of my favourite games, but this one is in 3D. Explore a virtual world by building with simple blocks, and fight monsters at night. I have only played "Classic Minecraft" since the full version cost $26. I plan on buying it soon.
I have been using this free program for at least four years. It has so many uses. You can draw everything you can imagine with it in a 3D environment and then move around the object like you were there. I've used it for planning projects, modeling random things for fun, and modeling 3D buildings for Google Earth. You can view the models I've uploaded here.
This program gives you a virtual globe to navigate. You can view mountains and terrain, 3D buildings made in Sketchup, trees, and it's integrated with street view. It also has a built in flight simulator, so you can fly around an accurate 3D globe.
This free software lets you broadcast a webcam for the world to see. It's pretty easy to use once its set up. My webcam uses this software to run. Find out how to see it by going to the Webcam tab above.