Magento has a great feature to merge all CSS/JavaScript files together into one big CSS and JS file. However this feature has one significant drawback. The browser caches the resulting combined file and does not refresh that cache after you update any of the CSS or JS files.

The new Activo CSS/Javascript Versioning extension for Magento allows site admins to assign version number to their CSS and JavaScript, so the name of the combined file will change and browser will reload the style sheet and/or JavaScript file.

Find more details at the CSS/Javascript Versioning extension product page.