Editors for Linux
- - - - - - - - - 
vim
emacs
pico
kate
gvim
...

Editors for Mac
- - - - - - - -
same as linux
TextWrangler - http://www.barebones.com/products/textwrangler/
TextMate - http://macromates.com/ - NOT FREE, BUT GOOD
Xcode
...

Editors for Windows
- - - - - - - - - -
Notepad
MetaPad - http://liquidninja.com/metapad/
UltraEdit - NOT FREE
...

In general
- - - - - -
http://en.wikipedia.org/wiki/List_of_text_editors