There are hundreds, however I am in love with PANDORA.COM - it is far from perfect, but if you tell the site the songs, bands etc you like then they play that kind of music continually - no adverts and it's free. You cannot download the songs, or replay them again and again or expect to hear specific songs - I have 'found' loads of new bands this way and because you can choose dozens of different 'stations' you can change the type of music you like all the time. As for 'alternative' - I have stations set up to find music like Tool, Default, Breaking Benjamin, 40 below summer and Coheed and Cambria - always hit and miss, always interesting and if you dont like a song, you can skip it (up to a maximum of 10 skips an hour) - Enjoy!