A traditional UNIX system text editor, should be on any UNIX system. It requires learning a few key combinations, but is very powerful, and it is also quite small. vi is well known for its minimal use of resources.
vim - vi improved. A newer version of the vulnerable vi editor. Many systems use vim rather than vi.
More than just a text editor. This text editor has a steep learning curve but is also very powerful, it is both advanced and quite large. emacs can do anything, surf the internet, chat, play games and many other tasks.