TRAMP. Being able to seamlessly remotely edit files and run remote modes. At work I regularly edit files on 10-15 remote machines. But I never ssh to them and run an editor. Instead I'll just visit them through TRAMP, run Magit to commit stuff that I've done etc.
My coworkers often don't even realize I'm editing a remote file or working in a remote shell. It's really that seamless. Of course, sometimes I forget I'm working remotely, and hilarity ensues. The worst episode was in a recent school project. I thought "oh, I'll just finish this on the train". And then, when I got on the train, realized I had actually been working remotely. Oops.
Not only is it seamless editing, you can actually use tramp to "cd" into a directory on a remote machine using eshell.