"SecondLife" is the Online-World by Lindenlab, I think you used it before.
"OpenSim" (Open Simulator) is almost the same, but opensource. You can have your simulation online (you can have visitors or "hypergrid" to visit other worlds) and offline.
"Sim-on-a-Stick" is almost the same as OpenSim, but completely independent because no installation is needed (but I have to admit that it is not as fast as located on a harddrive)
The difference to SecondLife is that you have access to the console, any upload is free, you can work offline ( important for schools and universities) and more.
You can use the simulation in many ways and for many purposes:
- simulate architecture
- present art
- creating filmsets and even illustrations
- applied science, e.g. learn mathematics by creating
- learning languages (e.g. when you have your simulation online and german and italian students build together and exchange their expieriences
- a friend of mine teaches children who are not able to move like the others, they get a feeling of freedom ...
- and and and
Crista Lopes is one of the main developers
http://metaverseink.com/blog/I forgot to mention that my sim-zip only works on windows.