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.
If you do a search for a program called Keyword Rockstar, it will analyze keywords, competitor sites and create PPC campaigns for you. I watched the video of it yesterday with Joel Peterson. The down side is the price, almost 1K. Ouch!
The powerful keyword research software never compromises its user friendliness. You can make money online even if it is packed with plenty of useful features, it is simple to use and navigate. It organizes your marketing campaigns simply. This feature is extremely useful, if you are organizing campaigns for your multiple websites. If any software requires you to spend more time in figuring out how to run it, it may not be the reliable one.
It's also important with this type of tool to make sure you get proper support from the developers. Keyword tools generally require more upgrading over time and require an attentive support team. If you have any troubles, you should not be left confused. It is wise to look for software that provides a handy user manual so that you can refer it at anytime if you face some problems.
[edited by: jatar_k at 2:37 pm (utc) on Apr 29, 2010] [edit reason] no urls [/edit]