Brackets (backed by Adobe) and Atom (backed by GitHub), are two open-source code editors vying for the spot that Sublime Text is leaving in the market due to its glacial development rate.
And they are both interesting in that each uses traditional web development languages (HTML, CSS and JavaScript) to both create the interface, and to provide an API for developers to use to customize. Do you need to modify or create a syntax color scheme? CSS. Do you care to create a plug-in? JavaScript. Use the languages you already know as a web developer.