Pandora: is a flash app that encourages you to create your own internet radio station. You "train" your radio station by saying which songs you liked or disliked, and the radio station goes out and finds new music that matches your preferences. (The preference-tracking algorithm is surprisingly good.) You can create a variety of different radio stations, and also share your radio stations with your friends.

The only catch is, you can never rewind. If you want to hear a song you liked again, Pandora provides a handy purchase link to Amazon. What a brilliant idea for a money-making web service!