Continuing this summary of currently available Virtual World platforms and engines, my final five are among the lesser-known, but do check them out.

Alice
Alice is an innovative 3D programming environment that makes it easy to create an animation for telling a story, playing an interactive game, or a video to share on the web. Alice is a freely available teaching tool designed to be a student's first exposure to object-oriented programming. It allows students to learn fundamental programming concepts in the context of creating animated movies and simple video games. In Alice, 3-D objects (e.g., people, animals, and vehicles) populate a virtual world and students create a program to animate the objects.
http://www.alice.org/

NeL
NeL is a toolkit for the development of massively online universes. It provides the base technologies and a set of development methodologies for the development of both client and server code.

NeL contains the following libraries:

• Miscellaneous library
• 3D engine
• Network engine
• Sound engine
• Collision engine (PACS)
• Form management (Georges)
• Landscape management (LIGO)
• Logic engine

NeL works on Windows, Linux and Mac under the GNU General Public License.
http://dev.ryzom.com/projects/nel/wiki

OpenSceneGraph
 The OpenSceneGraph is an open source high performance 3D graphics toolkit, used by application developers in fields such as visual simulation, games, virtual reality, scientific visualization and modelling. Written entirely in Standard C++ and OpenGL it runs on all Windows platforms, OSX, GNU/Linux, IRIX, Solaris, HP-Ux, AIX and FreeBSD operating systems. The OpenSceneGraph is now well established as the world leading scene graph technology, used widely in the vis-sim, space, scientific, oil-gas, games and virtual reality industries.
http://www.openscenegraph.org/projects/osg


Solipsis
Solipsis  is a French R&D project (ANR-RIAM) leaded by Orange Labs, funded by ANR and Media & Networks cluster of Brittany, launched in January 2007 and based on prior works dating from 2000. Five partners are involved:

• IRISA - Peer-to-peer networks & distributed systems
• Archivideo - Auto generation of 3D models & declarative method
• Artefacto - Avatars, contents & enhanced 3D modelling tools
• Rennes 2 University - Sociology of Community
• Orange Labs - Navigator, node and a bit of everything

Solipsis aims to create a public, massively-shared and user-generated unbound digital universe, sustained by a dedicated Peer-to-Peer protocol, with a modern day rendering engine and some great and accessible 3D modelling tools. In other words: a decentralized Metaverse platform. 

http://www.solipsis.org/

 

Syzygy
Syzygy is a programming toolkit for writing virtual reality or other graphical applications. Syzygy applications can run on a single computer, but it is especially designed for the creation of applications to run on clusters of networked computers. Programs or instances of the same program running on different computers in the cluster communicate with one another and share data.
Syzygy runs on Windows, Linux, MacOS X, and Irix. A cluster can be heterogeneous, i.e. you can mix different operating systems. Installation varies somewhat between operating systems.
The Syzygy libraries themselves are written in C++.

http://syzygy.isl.uiuc.edu/szg/index.html