Planes of Existence
The Planes of Existence was a series of 10 different telnet-based chat forums created by internet aliases Cat (real name Adrian Meredith) and Sootie (real name Selina Kubach) in 1996. It ran under them until 1999 when Sootie gave up her position and it was taken over by hackers from the rival site Crystal Palace, who continued to run a modified version of the site until 2002.
The main aim of the site was to combine popular "adult" themed chat sites with "general" chat sites using a philosophical model conceptualised with the metaphysical concept of the Planes of Existence, whereby many different worlds can exist concurrently and you can access any of them from a portal that connects them all. This concept is popular in many Eastern religions and the idea is that both "earth" and "heaven" exist as different planes of existence, but there are many others. Thus by using this model it was believed that all idealogies would be accepted, so long as they were clearly separated by the portal.
The adult themed sites were made popular by disgruntled members of "multiple worlds" talkers, such as Lintilla, Sleepy's Worlds and Fantasia's worlds, who saw Planes of Existence as a viable alternative to the other 3. The general themed sites were never particularly popular. While Planes rivaled both Lintilla and Sleepy's for popularity, overall it was never quite as popular. It was nonetheless much more popular than Fantasia's and the others. It was never close to as popular as the more popular general themed worlds, such as Crossroads and Resort. Later, the adult-themed talker Crystal Palace, who had orchestrated the hacking to take over the site in 1999, encouraged users of this site to move to Crystal Palace instead, finally closing POE's doors in 2002 with very few users left. Crystal Palace, in many ways the later version of this site, eventually became the most popular chat site in the world.
While the site has not existed since 2002, and only lasted for 3 years in its original form (6 years total), it nonetheless holds a vital role in the history of talkers, which in turn were a part of the internet's evolution towards modern forms of internet communication.