Ask HN: What do you use to organize your team knowledge?

Last year, me and three good old friends of mine founded a small web/mobile development team. Things are going pretty well. We're learning a lot, and new people are joining the group.

Keeping knowledge always updated and in-sync is vital for us. Long emails threads are simply not the way to go for us: too dispersing and confusing, and hard to retrieve after a while.

How your team manages and organizes common knowledge? How do you collect and share useful resources (articles, links, libraries, etc) inside your team?