Google AdWords Keyword Tool
(Category: Tools and Books)
I haven't used Google's AdWords Keyword Tool that much, mainly because I'm not an active AdWord user. However, I just learned that this tool can help to know what Google thinks your website is about. Below is the URL of that tool:
With this tool, you can type in the URL of your web page and click on "Get keyword ideas". The result is what Google's LSI (latent semantic indexing) spider thinks your page is about. It's also telling you the kind of AdSense ads that it is likely to place there.