I think mp3Skull.com is the best one. and i would like to suggest you all use MP3 Music Player called "Muze" for iPhone. Muze let you download all website above and listen to the downloaded mp3 with the songs in iPhone Music Library. No need to sync!!! here is more information
EZ-tracks.com is also another free website that you can use to download music. It also offers instrumental karaoke versions of songs for free. You can also check out Beemp3.com, Abmp3.com, Music-Oasis.com as well as ArtistDirect.com.