Probably the most notable hacker community is the community of open source/free software programmers. In this community, Richard Stallman and Linus Torvalds are two of the most well-known hackers.
People contribute to such a community for various reasons, like making useful contributions where they can, wanting to replace proprietary software with open code, or being a part of a larger group.
The Internet plays a key role in hacker communities; it allows people from around the world to collaborate on a project.
In a sense, Wikipedia can be viewed as a hacker community.