Google Webmaster Tools 5

If you own a website, Google Webmaster Tools is absolutely invaluable.  It lets you know how often Google is spidering your site, how deeply, and if it runs into any problems.  My favorite part is the content diagnostics, which shows you where you have duplicate meta data.  Basically, Google Webmaster Tools takes the guesswork out of making your site as Googlebot friendly as possible.  A fantastic tool.