Google's New Robots HTTP Header: X-Robots-Tag

Google has recently added a new way to block Googlebot—this time with an HTTP header called X-Robots-Tag:.

Examples:
X-Robots-Tag: noarchive
X-Robots-Tag: noindex, nofollow

Useful for blocking content like PDF files that can't have meta tags attached and that still might be in the index (because Google often lists pages that are blocked with robots.txt, sometimes even indexing them fully).

Watch out for this because people may try to scam you when linking to you. It may look like the page is not blocked with robots.txt or blocked with a robots meta tag, but they still could be blocking the links on that page with an HTTP header like this:
X-Robots-Tag: nofollow

Useful Google Robots Resources

List of Google's robots
Intro to controlling robots
Google and Robots.txt