Ever wanted to create a virtual world?

Now you can with OpenSim. Check this out: CLICK 

Features:

• Supports online, multi-user 3D environments as small as 1 simulator or as large as thousands of simulators.
• Supports 3D virtual spaces of variable size within one single instance.
• Supports multiple clients and protocols - access the same world at the same time via multiple protocols.
• Supports realtime Physics Simulation, with multiple engine options including ODE.
• Supports clients that create 3D content in real time.
• Supports inworld scripting using a number of different languages, including LSL/OSSL, C# and VB.NET
• Provides unlimited ability to customize virtual world applications through the use of scene plugin modules.

Minecraft

Minecraft is a construction-based online virtual world for all ages. Walk through the virtual space and survive dangerous monster attacks and environmental hazards. Do this by gathering blocks and building block-based creations. The players can also pick up food, tools and weapons to accomplish other tasks in the world. Or just enjoy building. VERY POPULAR.

Wizard101

Wizard101 is a massively multiplayer online game where children take on the role of wizards while playing this game over the internet. If you love Harry Potter you will love this game too!

Second life

Yes, this game isn't new. But it is classic, i quess. Give it a try. This virtual world is the most popular one.

Second Life is a 3D world where everyone you see is a real person and every place you visit is built by people just like you.