I'm not sure who suggested this, but I've added a feature that shows which songs are already in your cart.
To use it, simply add a song to your cart by clicking the Purchase button. Then, when you browse for more songs (for example, you click the "Continue Shopping" button to return to your previous search...), any song that is already in your cart will have the "In Cart Already" button appear. If you click it, it will simply bring you to your shopping cart.
This is a handy feature to help you know which songs you've already listened to. Also, even through it isn't a new feature, you can still use the "Add To Favorites" star to do something similar. Try both features out!