Recently I saw a strange comment on a website:

<!--googleoff: index-->

After a short google query, the comment can be traced back to the google seach appliance. The comments change the behaviour, how the page is beeing indexed. Eg: You can avoid that a part of a website is beeing displayed as a snipped in the search results:

<!--googleoff: snippet--> this text will not appear as a snippet <!--googleon: snippet-->

For all googleon/googleoff control abilites check out the google documentation I find it quite interesting, that google already parses html-comments for special control sequences like the Internet Explorer. Maybe this kind of google-coding to optimize your homepage will become available to the public google search in the near feature.