Text Editor
Most administration on a Unix-like system is done with a text editor editing configuration files, usually under the /etc directory. This document will just outline a few editors to choose from.
vi
vi (home is http://vim.org) can reliably be expected to be installed on just about any machine or rescue console. More at Vi
emacs
joe
joe is Joe's Own Editor. This editor uses a minimal amount of your screen, similar to vi, but is a more user-friendly editor that allows a user to position the cursor with the arrow keys and type. Ctrl-K-H can be pressed at any time to get a listing of additional commands available.
nano
nano is the free software implementation of pico which is a popular editor for new users. Usage should be pretty clear as the common commands are all printed out on the screen for you.
TextPad
TextPad is a free, powerful, general purpose editor for plain text files that works in Windows.