CSS Lint. It's an open-source online tool to check for typos, bad practices, incorrect properties for rules, inefficiencies, and other potential problems in your code.

I pasted in the primary style sheet I use for my work website. CSS Lint returned one error and 173 warnings.  The error was a missing colon in one selector. As for the warnings, they could be grouped into the three main problem areas: using IDs in selectors, broken box models, and qualified headings.

It's an instructional and helpful tool, especially for lengthy style sheets that have been used and abused for years. While you may not need or want to take action on every warning, CSS Lint will help you write better code moving forward. Users are welcome to contribute new rules to the tool.