Despite the massive adoption of smartphones, most business web sites are still primarily designed with computer users in mind. Google's GoGoMeter simulates how your site might appear on a mobile phone and identifies areas where you could improve.
While you can get a rough idea of how your site renders on your own phone, the advantage of the service is that it also provides specific suggestions on how to improve the site. With Lifehacker, it recognised that there's a system in place to recognise mobile content and offer a different experience on phones, and the main recommendation was a more visible search feature. With my personal blog, it rightly reminded me that I needed a better mobile theme and that my loading time was slow.
Obviously, you need to take the recommendations (offered up in a PDF report) with a grain of salt given the source. But if you want some easy wins to make your site more mobile-friendly, it's not a bad place to start. (And bonus cheeky points to Google for referencing a Yellow Pages ad campaign in the product name.)