The Adwords Insider Blog has announced the launch of a new keyword tool for advertisers - the "search-based keyword tool".
Interestingly, it will give keyword suggestions based on spidering data (you supply a domain name - it matches keywords within title and on-page keywords of URL in that site). If you use it for sites within your Adwords account, it won't suggest keywords you're already using.
From an initial play around, it looks like it will be very useful for campaigns that potentially have lots if missed opportunities for good paid keywords - particularly for larger sites where there may be product pages and the like that are easily overlooked. Spreadsheet export is nice too.
Data returned for sites you don't have Adwords for seems pretty screwy, unfortunately ;)
The tool's located at http://www.google.com/sktool/