Brackets is entirely free and open source.

Brackets will automatically refresh the browser and load the latest saved version of a file open in the browser. Works with php as well. Editing css will even highlight the tag that's currently worked on. It only works with Chrome though.

Hovering over colors hex codes or image paths will pop up previews of corresponding colors and images.

Functionality of Brackets can be extended via a simple to use extension manager. The extension manager also has a considerable number of extensions and themes.

The editor is built using html, css and javascript making it extendable by any front-end developer.

Brackets is being actively maintained and developed.

It is easy on the memory usage and starts quickly.

Default extension allows extraction of PSD resources, such as images and styles. Good integration for placing extracted resources into source.

A feature called quick edit allows selecting a tag in, for example, an html file and edit the associated style without switching over to the css document. It also supports SASS and LESS pre-processors.