Emacs

emacs written by Richard M. Stallman, is a text editor on crack. It has many modules -- sort of like plug-ins -- that allow you to do all sorts of stuff while still editing (e.g., checking your email.) It is difficult to learn to use for most, but those who do often grow enamored of it's versatility.

Other text editors: vi, joe, ee (aka edit), nano and pico.