Señor Mono

If three out of three platforms isn’t enough, you might want to go with vim. I guess it is ported to all platforms available.

Sublime is a text-editor on steroids. It has so many good extensions, it feels like an IDE.

Anyhow: paying for good software is a no-brainer, if it safes you troubles and time, and especially if yourself are a dev, too (depending on others to also pay for your work). Also there are fair company licenses in case a firm is involved.

And finally: you can use sublime without paying. There will he a pop up dialog every 50 start or so. It’s really not annoying and fair.

Sublime Text. On any platform. Nimble, mighty, extendable.