Ask HN: What non-email tool do you use for internal discussions?

Where I work, we have too much noise in our inbox. Consequently, it's difficult to get people to concentrate on the more-involved discussions because they are too distracted with calendaring meetings and booking travel.

If you aren't using a primarily email-based tool (like a listserv), what do you use to facilitate in-depth, asynchronous discussions on internal and confidential issues?