Twinity is probably one of the most interesting contenders in the realm of "full immersion" 3D worlds. At least it is certainly the most ambitious German project in this market. Metaversum, the company behind Twinity, is a firm believer in the mirror worlds idea (i.e. replicating real world locations in a virtual 3D space). They intend to reproduce some of the best know urban areas of the world first, starting with Berlin - not a huge surprise with a company based in the German capital. (Its fairly interesting to compare their ideas with the concepts behind newcomer Near btw.)
The platform is still in a closed beta currently. Client and servers are relatively stable, already, though. It can be loads of fun exploring the virtual Berlin - especially if you know Berlin in physical reality.
If you want to give it a try, just click HERE (the link is good for 50 sign ups; so please don't be disappointed if it won't work anymore in a few days).
WARNING: The Twinity client software is only available for Windows, currently. Yes, no Macintosh and no Linux version, either.