1999-2002: Linden World, and "the rig"
Linden Lab was founded in 1999, and for a couple of years didn't really have a lot of direction. One of its most convincing forms by 2001 was as a hardware-research company, focusing on haptic technology, which would allow commercially viable full-body access to virtual environments and waldoes (the aggregate slang name for the category of real-time telepresence and teleoperation devices).
The haptic hardware prototype was dubbed "The Rig".
The thing is, you can't really build a haptic rig without some sort of way of testing it, and a basic virtual environment had been developed around the same time.
Linden World, as it came to be called, had an ecosystem of rock-eating birds and bird-eating snakes. The original crude avatars (called Primitars) could fly (because nobody much cared to make animations for climbing up things), and could change the shape of the terrain by lobbing grenades. It even had weather.
Linden World was spread almost seamlessly across multiple servers (albeit, only a couple), and it was envisioned that one day it might become a sprawling and distributed agglomeration of third-party servers. The streaming content architecture and protocols allowed people to create content and to participate in content creation in real-time -- without drowning their connections in data.
Nobody would fund Linden Lab. It was, at the time, considered laughable by virtually everyone with deep pockets. Then Mitch Kapor took a chance on Philip Rosedale and other investment followed.
Linden Lab at this time was envisaging some manner of game.
That notion went out of the window at a board meeting. While Rosedale and Cory Ondrejka were presenting to the investors, other Lab staffers were using Linden World's building tools, and an image of the world was projected behind the two presenters.
As Linden staffers created a scene involving a giant snowman surrounded by a small horde of snowman worshippers (truly a scene worthy of Bill Watterson), the attention of investors turned increasingly away from the presentation and towards Linden World.
That seamless, real-time, collaborative content-creation was "it" -- the defining quality that made Linden World unique, and everyone could see that.
In March 2002 (March of each year is technically Second Life's overlooked birthday), Linden World opened its doors as a limited alpha and its first user, Steller Sunshine, appeared. Sunshine was already a veteran of virtual worlds, which by 2002 were closing on 20 years old.
When the staff arrived at the office the next morning, they discovered that Sunshine had created a game.
It was on.
Getting from a limited alpha with collaborative content-creation to a commercial product wasn't an easy road. That went to Hunter Walk, late of Mattel and the Conan O'Brien show. This regrettably also involved changing the name of the platform and the rather regrettable "Second Life" was chosen instead, which was not popular among Linden staff.
It's unclear as to where "the rig" hardware is located at present. It was shoved in a box at some point and largely forgotten. Linden Lab hasn't been able to tell us what became of it.