tmux is a "terminal multiplexer", it enables a number of terminals (or windows) to be accessed and controlled from a single terminal. tmux is intended to be a simple, modern, BSD-licensed alternative to programs such as GNU screen.
Please see the release notes and FAQ (note these are for CVS HEAD).
The change log, up to CVS HEAD, is here, the project page here and mailing lists here.
Screenshots (from CVS HEAD)