splitvt - Splits a terminal window horizontally. Basically just the functionality in screen's regions. (Page Removed)
dtach - allows detaching and multiply connecting to a single program. Useful if you don't use any of screen's other features.
twin - Text mode windowing environment. (Error 404)
ratpoison - X11 window manager with a similar philosophy to screen; keyboard interface, prefix command for all window manager functions.
xmx - X11 program for attaching to an X session from various locations.
tmux - Terminal Multiplexer. Very similar to GNU screen, with some great additional features (e.g. ability to switch between sessions), but lower portability (e.g. no HPUX, no cygwin) (Error 404)