JS_Harris - 11:18 pm on Mar 25, 2010 (gmt 0)

The Google Adwords keyword tool can do this one site at a time, it wouldn't take long to write down the top words from each site.

I just wanted to add that the value of doing this would be in the knowledge gained and not necessarily in the list you end up with. If you spot a keyword that makes sense which you haven't used by all means include it on your page BUT... the dynamics of what make a page rank well goes far beyond keyword use so don't expect this change to make much difference if your site has other weaknesses.