Brackets is an Open Source text editor for CSS and HTML. Get it here: http://brackets.io/
Looks like it currently supports Mac, Windows and Linux, but many users have said that it requires an internet connection to function (Adobe says they are working on this, but no ETA).
Brackets also supports added plugins – for instance the new Extract plugin is useful for extracting CSS code from PSD files.
The Extract for Brackets (Preview) extension lets you pull out design information from a PSD including colors, fonts, gradients, and measurement information as clean, minimal CSS via contextual code hints. You can also extract layers as images, use information from the PSD to define preprocessor variables, and easily get dimensions between objects.
Brackets is new, so only a few plugins are available, but it is worth looking at. It comes with a default light and dark themes, ability to split screens, and a live preview (works only with Chrome browser for now).