Matt Haughy founded MetaFilter, a highly popular website in which members post the best things things they find on the web each day. In an article for Digital Web Magazine, he offers the following advice on building communities:

1) Make sure you really want to do this
Community websites take an enormous amount of time and energy in moderating, advertising, responding to users, and re-coding the site.

2) Have both a compelling idea and compelling content
Be specific, with a well-defined mission, topic, and audience, and focus on quality. Try to recruit the best contributers you can.

3) Seed content sets the stage
It's important to have good content to start with, and good interactions around that content. Start with a small group of people you know will provide a good example.

4) Create some basic guidelines and be as fair as possible
Lead by example by posting frequently and intelligently. Keep a high profile, and treat others the way you would like to be treated. If you see something happen regularly that you would like to stop, make it part of the rules, and make the rules easy to find. Be nice about enforcing them - dont be overly headstrong, or seem to take pleasure in it. Be fair and consistent.

5) Have a place to talk about the site, somewhere on the site
This allows for feedback, and helps to track issues. It also helps keep this discussion organized and the main site on topic

6) Spread out the work as much as possible
Have trusted friends work as administrators and moderators, and allow people to contribute to the development of the site itself. This helps the site to keep going when someone leaves, gets sick, or goes on vacation.

7) Deal with troublemakers as quickly and nicely as possible
Defuse things as soon as can. Start by emailing the person and gently asking if perhaps they didn't see the guidelines, or letting them know the preferred way to act. Dont sound threatening or patronizing. If that doesn't stop it, enforce a penalty of some sort, like taking away posting rights, and let them know why you are doing it, and what they can do to get the penalty lifted.

8) Highlight the good, recognize the work of others
Use a system that enables the community to vote on its favorites and give these favorites a prominent place on the site.