nano GNU nano
The terminal editor for when you just want to change one line in a config file. Started in 1999 by Chris Allegretta as a free replacement for Pico, the editor in the Pine mail client.
The entire design philosophy sits along the bottom of the screen: the shortcuts are listed at all times, and ^X means you can actually quit the program without searching the web. That isn't a joke but a real advantage — an inexperienced admin who lands in visudo does less damage in nano. Over the years it has gained syntax highlighting, regex search, multiple buffers, undo and soft wrapping, so enough for maintenance work but not for development. It's the default editor in Ubuntu and Debian, which makes it the first editor most people meet on a server.