Tmux is fairly new, BSD-licensed, and has a much smaller/cleaner codebase than screen. It's quickly gotten features that people have been struggling to add to screen for years. (It comes with dwm/xmonad-style layouts out of the box, for one.)

If you already use screen and are happy with it, it's probably not worth switching, but if you're starting new, I'd suggest learning tmux first. You might want to get a fairly recent version of the source, though - it's in the main OpenBSD tree now, and it has been improving pretty quickly.

If you use Unix & ssh at all, definitely learn one of them. Yesterday, if possible.

One of these days I really need to write about getting started with tmux...

Another alternative I just heard about is "nx" (though it's commercial). It can resume entire remote desktop sessions as well as single windows. I've played with it a little, but I think I like tmux better.

Original comment was something like:

"Obligatory 'Have you tried tmux?!?!'"

I deleted it because I thought HN might think I was trolling or something.