PEOPLE & PAPERS
Here are some people and papers that I've found to be particularly illuminating and thought-provoking. (Click on a person's name or image to read more by this author) 
Players Who Suit MUDs by Richard Bartle
Richard Bartle built some of the earliest MUDs, and his observations about player interactions should be required reading for everyonewho'sdesigning online gaming worlds - and online social enviroments in general.
Cyberspace is not Disneyland by Amy Bruckman
Amy is a longtime MUD researcher with a passionate interest in education. She's an Assistant  Professor at Georgia Tech, where she teaches game design and virtual community design, and focuses her research efforts on building and studying Constructive Learning Environments. 
Proving the Value of Online Communities in Business by Joseph Cothrel
Joe is the VP of Research at Participate.com, a consulting company that helps large corporations build and maintain online communities.
Identity & Deception in the Virtual Community by Judith Donath
Judith runs the Sociable Media Group at the MIT Media Lab, where she explores  issues of identity and society in a networked world. 
Social Dimenions of Habitat's Citizenry by Randy Farmer & Chip Morningstar
Habitat was one of the earliest virtual worlds, and many basic ideas about online community-building were first articulated in the paper above, along with another seminal paper by Chip and Randy called The Lessons of Lucasfilm's Habitat.
Randy Farmer
The Art of Online Hosting Good Conversations Online by Howard Rheingold
Howard write the seminal book, "Virtual Communities" and is the CEO of Rheingold and Associates, a company that helps organizations get their community up and running.
Howard Rheingold
Production of Trust in Online Markets by Peter Kollack
Peter Kollack is a sociologist who specializes in  studying the dynamics of online marketplaces. 
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The Laws of Online World Design by Raph Koster
Raph is a longtime MUD developer who currently designs online games for Verant Interactive, the company that created EverQuest. Raph was the lead designer on Ultima Online, and his personal web site contains many of his writings and musing on game design, along with stories, poetry and music.