While completely readable by machines, minified code causes headaches when interpreted by human beings; thus, prettifiers, or “beautifiers”, convert the code back into a human-readable format.
ProCSSor takes CSS code (either directly via copy-and-paste, as a file upload, or a URL) and cleans it up according to rules you determine; the default ruleset works well for most code you’ll encounter.
Styleneat is similar to ProCSSor, with fewer rules but the ability to sort the converted CSS code alphabetically by selector and/or property.
Code beautifier is an online tool based on the venerable CSSTidy application, with the added option to compress and optimize the code at the same time; converted code can be given a balance between readability and file size.
Syntaclet is a bookmarklet application for your browser which recognizes a wider variety of languages. The app is intended to do on-screen conversions of poorly-formatted code examples on web pages.