Ask HN: What's the best way to create a community-driven site?

I tend to have a lot of ideas for community-driven sites. Things like Dribbble or StackExchange but much more niche.

While I have experience building those communities from scratch, setting up a new database and creating the necessary scripts is a little more time consuming than ideal.

Are there any open-source or pre-packaged solutions for creating community sites (registration, database management, etc.) quickly?

Think of this as a question similar to: How can I regularly launch a new community-driven project every weekend?