Google created this tool to help Adwords advertisers find all the related keywords to target to their websites. But this tool can also be used by your competitors to see what keywords you are targeting on your website.

If the site is not associated with your Adwords tool, it allows only limited set of keywords (100 of them).

Go play with the tool and find out all those longtail keywords which your competitor is using to bring that extra traffic to his website. There are many possibilities here. Go explore and don’t forget to bookmark it.