What tools you’re using while building a community? #Mozcamp

I was thinking to talk about “10 apps you can use for community enchanting” during the MozCamp in Berlin next month. I will be thankful if you give me some examples what tools, except so called “main stream” like Facebook, Identi.ca and Twitter you are using to deal with communities and what is the purpose of it.

Here is an example:

Tool: Trello (trello.com)
Purpose: I am using it for internal project management within the community core
Reason: Easy to use

Thanks!