Now that Google Talk 188.8.131.52 is available to public, Google thought it's a good idea to use the songs listened by GTalk users for a new service: Google Music Trends. Every week, you'll see the most popular 20 songs by genre and by country. Currently, only US is an option in the country list.
The service is integrated with Google's music search, so if you click on the artist name you'll find the latest albums and other related information.
The new service is opt-in, so you need to enable it in Google Talk. Go to Settings / Audio and check "Share music listening history with Google Music Trends".
Another interesting side effect is that all the songs played will be included in the Search History, so you can see what you listened last week or how many times you listened to U2. Of course, to record that information, you need to use Google Talk and show the current music track in the status.
A similar ranking can be seen on last.fm, the site that uses plug-ins for music players to send information about the listened songs. The site also shows artist charts, top tracks for an artist and much more. But remember that popularity isn't a measure of value.