OS X: Sublime Text 2 - really nice syntax highlighting and good function referencing. Also supports session saving and function collapsing. (vim and joe on the command line as well)

Win: Notepad++ - pretty much the same thing as Sublime in the reason I use it.

X11: I really don't have one, I use a cli editor usually Possibly gedit if copy/paste is being fickle. I think if I used an OS with X11 natively again I'd give the gtk version of vim a try.

Unix CLI: Vim - I used to be against the VI based editors, but recently I started using VIM in a Linux admin class and the power of it's syntax coloring started me to give it more a shot. I am far from a "vi ninja" but it's replaced my main cli editor.

CLI Alt: joe - it's a very powerful dos-eque editor that has most of the features of vim. It's an easier learning cure. I still use often.

LibreOffice is another full word processor/office suite (actually it's another version of open office) as well. I think this version gets the more up to date stuff. I do use Word at work as well as the rest of office being a MS University. I tend to keep it plain text for my stuff most of the time.