When I first thought about doing an article on this topic I was going to call it speculative, but Internet reality has caught up already.
Previously, insufficient computational and communication capability have been the main limits, not the imagination of programmers. With Second Life by Linden Lab we seem to have crossed a critical boundary and reached a point where large numbers of users can share the same virtual world at the same time. Major corporations are experimenting with a presence in the virtual world as a means of reaching customers and individuals are creating businesses which exist only online.
Downloading the Windows XP or Mac client and creating an "avatar" identity with basic membership is free. "Owning" or "renting" virtual property so you can establish a more extended presence involves a monthly fee.