I think GNU/Screen should get some sort of award for being a really cool piece of software. Or maybe it's Unix Ptys that should, but screen is what really exploits them...
The multiuser feature is something that I use rarely, but when I do, I really appreciate it. Today, I used it to help diagnose a co-worker's problems with building some software, from the comfort of my desk, without having to try and squeeze in next to him at his desk, and squint at his ridiculously small xterm font.
I've previously used it to train another sysadmin, so rather than two of us having to sit on each others laps in front of one computer, we could both sit at our own desks and talk to each other, while taking turns to drive the same shell.