If you love music, you love this website that lets you track the changes in popular music, with the ability to listen to all of the popular songs through the decades. And it's free.

From Bing Crosby and the Big Bands to Justin Bieber and Katy Perry, the Radio Time Machine website gives you an overview of popular music from 1940 to the present time. The trip is made via a great interface that lets you see the album covers and listen to a clip from each song. Better yet, if you want to hear the full songs, all you have to do sign-up for a free Rdio account.