Cyburbia calls itself an "urban planning portal," a resource for urban design students, professionals, and self-described "planning geeks." Since that last phrase definitely describes many of us here at WorldChanging, Cyburbia is a worthy addition to the worldchanger's bookmark list. I expect to be returning to it often.
Cyburbia is a classic web portal -- it indexes and links to thousands of planning and urbanism-related websites organized by subject matter and focus. The site also houses active web forums on urban planning, as well as photo galleries of both the best and the worst in city design. While sustainability and sustainable urban design are not the focus of Cyburbia, they are part of the bigger picture embraced by Cyburbia.
Cyburbia is a volunteer effort, built and maintained with no profit-making intent. It's been around, in various forms, for nearly a decade. While it's not the biggest planning-related site around, it's arguably the best. If you have the slightest interest in urban design, it's definitely worth adding to your bookmarks.
A friend who's an urban planner at Urban Land Institute says this's one of many good sites, but the favorite of him & his cronies is Planteizen: http://www.planetizen.com/
Jer, Planetizen seems to be one of the contributors to/sponsors of Cyburbia.