Google Webmaster Tools

If you run any sort of public website (a blog, a corporate site or whatever) I would recommend having a look at Google Webmaster Tools. Basically if you insert a tag in your site's homepage header as verification that you own it, Google will give you access to a whole bunch of their database stats and information to allow you to see in quite some detail what you are being searched on, which pages are doing well or badly, the pagerank of different pages in your site, errors that Google has encountered while spidering your site and much much more.

Definitely worth a half hour peruse.