A Modern, Extensible & Free IDE – KineticWing
KineticWing is a free IDE that is lightweight, portable and works on all major operating systems (Mac support is mentioned to be coming soon).
It is not a complicated IDE but more like a smart text editor with modern features.
These features include support for Sass or Less (with built-in compiler), HTML5-CSS3 or CoffeeScript.
KineticWing is supporting server-side languages as well and more of them are being added within time.
The editor is not yet perfect, it is in beta status, but the features and roadmap are very promising.