I recently discovered Tmux after years of using GNU Screen. I love it. It feels much easier to use and more intuitive, easier to script. Also integration with terminal emulators such as iTerm2 on OS X or Byobu on Linux is fantastic. Tmux earned a place in my list of favorite daily used tools.
The biggest advantage of tmux over screen is its ability to automatically resize based on the smaller terminal connected to it. I know there are a lot other advantages, but this in itself was a game changer for me.
