Clearly the best OpenSource editor. If people say it's too complex etc; that's partially true but it's worth learning it. Really, really powerful and you can script it in many languages including ruby and python.
Here some links if you're new to Vim or want to find out something new:
I just can't say more. It just terrible rocks. I use it for all the documentation I do. And I probably will use it for documenting my ruby libraries too because there is no good ruby equivalent :(