From Sirikata Wiki
What is Sirikata?
Sirikata is a BSD-licensed platform for virtual world applications. Virtual worlds are shared, online, 3D, interactive spaces. They have a lot of applications -- from games and entertainment, to education, to social applications. Sirikata makes creating virtual worlds easy.
If you're not sure if Sirikata is, start by learning more on the home page or just try it out by following the instructions below to login.
How to get started
Sirikata provides all the tools for creating a virtual world, but it isn't a single world that you can log into. Instead, you use Sirikata to run your own world or to log into someone else's world to interact with others and write scripts.
If you want to join a world, start out with the Scripting Guide. It'll help you get connected to a world and write some simple scripts to make objects in the world do some simple things. Almost everyone should start here.
If you're an advanced scripter, check out the Architecture Guide to better understand Sirikata's architecture, it's features, and it's limitations.
And if you're interested helping build Sirikata itself, look at the Platform Development Guide to help you take your first steps into the Sirikata source code.