ArduoCSS is a Css editor with an intuitive and easy-to-use interface.With ArduoCSS you can preview dinamically how html is affected by your stylesheet changes. Most commons css properties can be choosed by drop-down menu, reducing the typing mistakes.
Freeware application. It's using open source components and is on SF, an open source host, but the Files section only has EXE setups. That is not open source.