Last week, Google introduced a format to publicly specify the preferred URL for a page. You simply add this link tag to any page:
<link rel="canonical" href="http://www.example.com/page/" />
The "canonical" attribute tells the Googlebot which URL is the preferred address for any given page. This is perfect for me with a site like Silent Uproar. We've changed our URLs several times over the 8+ years the site has been running and Google has cached 2-3 different URLs for a lot of our archived content (i.e., some with a PHP extension, query strings and/or mod_rewrite URLs).
Not everyone is going to be able to utilize this update, but Google has already fielded a lot of questions for the people that will implement it. It's definitely a nice thing to have for sites with a bit of history and duplicate content issues.