Lets look at this in two steps:

1. Virtual Worlds

2. Serious Virtual Worlds

"A virtual world is a computer-based simulated environment intended for its users to inhabit and interact via avatars. This habitation usually is represented in the form of two or three-dimensional graphical representations of humanoids (or other graphical or text-based avatars). Some, but not all, virtual worlds allow for multiple users.

The world being computer-simulated typically appears similar to the real world, with real world rules such as gravity, topography, locomotion, real-time actions, and communication. Communication has, until recently, been in the form of text, but now real-time voice communication using VOIP is available. This type of virtual world is now most common in massively multiplayer online games (Second Life, Entropia Universe, The Sims Online, There, Red Light Center), particularly massively multiplayer online role-playing games such as EverQuest, Ultima Online, Lineage, World of Warcraft, or Guild Wars and the massive online multi-player game RuneScape" via Wikipedia

A serious virtual world is as above but specifically built for non entertainment purposes such as education, business collaboration, disaster planning and would generally be used in the professional space rather than the consumer space.

This is not to say that virtual worlds such as Second Life cannot have serious activities occuring within them. But, by definition, their primary purpose is entertainment. 

At this time there is a single example of a toolset for building and deploying serious virtual worlds: Forterra