What is Last.fm?
Last.fm is a site dedicated to music fans. It compiles charts and recommendations based on what you listen to. All you need to do is sign up, install the plugin on the mp3-player you use and you're good to go. You also have an option of downloading their player and listening to various audio streams provided for free.
Here's an example user page of what kind of data you get from the site: Example charts
A tour of the site and more detailed information: Here
How does this relate to Battlemaster?
You can form groups in Last.fm. Various charts are then calculated for the group, based on what the members of the group listen to. As such, there is a group dedicated to Battlemaster (link). Basically, what does the Battlemaster crowd listen to? Join in, spread the word and help the charts get more interesting. Post in the groups forum and get a discussion going about anything music related.
Active participants always wanted :)