Being freely available from the start, even while closed source, helped a lot in Blender's adoption. A large, stable and active community of users has gathered around Blender since 1998.
The community showed its best in the crucial moment of freeing Blender itself, going Open Source under GNU GPL in late summer 2002.
The community itself is now subdivided into two, widely overlapping sites:
The Development Community, centered around the Blender Foundation site http://www.blender.org/. Here is the home of the development projects, the Functionality and Documentation Boards, the CVS repository with Blender sources, all documentation sources, and related public discussion forums. Developers coding on Blender itself, Python scripters, documentation writers, and anyone working for Blender development in general can be found here.
The User Community, centered around the independent site http://www.elysiun.com/. Here Blender artists, Blender gamemakers and Blender fans gather to show their creations, get feedback on them, and ask for help to get a better insight into Blender's functionality. Blender Tutorials and the Knowledge Base can be found here as well.
These two websites are not the only Blender resources. The Worldwide community has created a lot of independent sites, in local languages or devoted to specialized topics. A constantly updated listing of Blender resources can be found at the abovementioned sites.
For immediate online feedback there are three IRC chat channels permanently open on irc.freenode.net. You can join these with your favorite IRC client.
The IRC channels are #blenderchat, #blenderqa and #gameblender. The first of these is accessible even without an IRC client, using a plain Java enabled Web Browser through the elYsiun site (http://www.elysiun.com/).