Google Webmaster Tools (GWT) is the primary mechanism for Google to communicate with webmasters. Google Webmaster Tools helps you to identify issues with your site and can even let you know if it's been infected with malware (not something you ever want to see, but if you haven't spotted it yourself, or had one of your users tweet at you to let you know, it's invaluable).
And the best part? It's absolutely free. If you don't have a GWT account, then you need to go get one now.
This guide to Google Webmaster Tools will walk you through the various features of this tool, and give you insight into what actionable data can be found within