The tools listed here can make your CSS better, faster, and more efficient.

SlickMap CSS – A Visual Sitemapping Tool for Web Developers
A tool to create a styled sitemap from a basic HTML unordered list.

Phoenix : Add-ons for Firefox
Phoenix is a Firefox add-on editor that lets you work on CSS, HTML, and JavaScript code, including testing. It also tells you how many CSS or JS files are loaded into a page, along with their size, and lets you control those files.

Juicer – a CSS and JavaScript packaging tool
This app will combine your multiple CSS or JS files into a single file, reducing HTML requests and speeding up load times.

Gridmaker v3
A simple online grid-design app.

Codetech Firefox Extension
Get the feel of Dreamweaver in a Firefox extension. Edit your documents right next to your web pages as you surf.

Boks – a visual grid editor
Boks is an AIR application (so it works on Windows, Mac and Linux) that provides a User Interface for Blueprint CSS’s framework. It’s been designed for those who think the Grid System is good but never really took the time to give it love. It handles grid configuration, baseline rhythm pimpin’, CSS (with or without compression) and grid.png export and HTML layout.

Modernizr
Modernizr lets you create if-statements in your CSS with an intuitive syntax.

CSS Vertical Rhythm Generator
This tool will help you compute CSS that has a consistent vertical rhythm. If you are not sure what this is all about check out this article about vertical measure.

Typeselect – Selectable text replacement
By leveraging typeface.js, jQuery, the canvas, toDataURL, CSS background properties and real overlayed text, Type Select is able to combine custom fonts with your browser’s native text selection funcationality. You can now interact with beautifully rendered typefaces just like you do with normal text. Drawbacks: no support for :hover or line breaks, text is not selectable in IE.

CSS Prism
Enter the URL of any CSS file to view and modify its color spectrum. You can use CSS Prism bookmarklet to view and modify the color spectrum from any website.

Compress PHP, CSS, JavaScript(JS) & Optimize website performance.
This articles gives a good overview of GZip compression.