TRAMP is a tool for Emacs to let you edit files on another computer, using only an ssh connection. It rocks, as it lets me edit files at work from my laptop at home. Behind the scenes, there’s some serious voodoo going on, encoding and decoding files on the fly, transferring them to the appropriate place and so on. But it’s all 100% transparent.
But what’s really made my jaw drop is that it integrates correctly with vc-mode. I have a file that’s checked in to RCS on the remote machine. Without thinking I hit the checkout button to start editing the file. And it worked! I totally wasn’t expecting that. But it’s damned useful.
In summary, if you use Emacs, you need TRAMP.