There are great and popular CSS preprocessors like SASS and LESS.
Myth is an other one which allows us to write pure CSS without having to worry about browser support, or even slow spec approval.
It simply functions like a CSS polyfill but we can still use variables and math functions.
Semantic is an exciting new front-end framework that comes with a set of specifications for sharing UI elements between developers.
The vocabulary used (for classes and IDs) is much clear compared to alternatives which lowers the learning curve.
There are styles for many HTML elements, UI elements and scenarios (buttons, forms, tables, accordion, navigation, commenting..).
LESS is used as the CSS pre-processor and Semantic is currently in a pre-release version but planned to be improved with much more stuff.
Web designers/developers who write client-side code with a language that needs to be compiled, usually lose some time going back and forth with their editors and the compiling interface (usually command line).
If you don’t already have a setup for auto-compiling, Koala might be what you need.
It is a desktop application that works on Windows, Mac + Linux and can compile Less, Sass, Compass + CoffeeScript.
Koala can compile the files manually or automatically once they are changed and compresses them.
Also, it is possible to set different options for each file to be compiled.
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.
When building websites with scripting languages that require compiling (Sass, Less, Stylus..), compiling them manually (or via refreshing the page) is usually an extra step.
Prepros is an open source application for Windows OS that can do that automatically for Less, Sass, Scss, Stylus, Jade, Coffeescript, Haml and Markdown.
It auto-compiles the source and injects the CSS (also HTML and JS) to the browser so that no refresh is needed.
There are options for the compiling process like the output format or its style (compressed, nested..).
Prepros also has a built-in + static HTTP server for testing any ajax requests.
Requirements: Windows OS
If you need a base when starting a new project and don't want it to be complicated, Kube Framework can be a good choice.
The framework is a single CSS file that brings responsive layouts with no headache and offers more.
Kube includes styles for forms, grids, buttons, tables, typography and few other stuff like links or images.
For developers looking for maximum flexibility and customization, LESS files are also provided.
Compatibility: All Modern Browsers
If you are one of the designers/developers who jumped into the LESS world to get more out of CSS, you will love Css2Less.
It is a web-based tool for converting CSS files into LESS by simply copy-pasting the styles.
The tool uses the css2less Ruby library in the backend and its website is open sourced for anyone willing to host it themselves.
Fiddle Salad is a fresh web development tool, an online IDE, for client-side web applications.
All the code is written in a single-page interface and the output is displayed/rendered instantly.
There is built-in code completion support for CSS + jQuery and links to the documentation for the properties of each language.
With a free membership, it is possible to create any number of projects and save them for re-working on them without losing any settings or data.
If you already use it, then you know that LESS is a "time and byte-saver" when creating + serving stylesheets (if not, you should definitely check it).
You either use the .less files directly (via less.js or on the server with Node.js) or compile them into CSS from command-line and use only the CSS.
Crunch is a free Adobe AIR application that eases the compiling part without the need of command-line but via a user-friendly interface.
It is a simple CSS/LESS editor with syntax highlighting and multi-tab support.