Unicorn combines a number of popular tools in a single, easy interface, including the Markup validator, CSS validator, mobileOk checker, and Feed validator, which remain available as individual services as well.
W3C is inviting developers to submit their own modules for the validator to continue to expand its capabilities. So far Unicorn has been translated (localized) into 21 languages, and is hoping users can contribute more.
Unicorn's CSS profile validation includes the ability to choose warning levels to report and to choose a medium (all, braille, handheld, print, screen, etc.). The "Custom Task" allows users to choose among (X)HTML, CSS, MobileOK, and RSS/Atom. With this custom option, users can also choose a level of CSS (Level 1, 2, 2.1, or 3) in addition to other profiles (SVG, mobile, TV, etc.) and then again from a CSS user medium.
The report page has a much nicer way to display all the issues than previous validators, allowing users to collapse an entire section and showing icons and numbers corresponding to the count of specific error types within a section (2 errors, 3 warnings, for example). Each message also shows the specific line number and (if appropriate) character number of the error (warning, info alert) along with the corresponding message. Each section and message even has an anchor on it so you can link directly to any item for sharing issues with a team. Now if W3C could add a background color to the page (white would be ideal), then I wouldn't have to squint to see the results.
Give Unicorn a try and see what you think.