Codename46

Hey guys, I was wondering what software programs exist that do a good job of extracting vocals from mp3s, or whether there are other practical means of extracting or obtaining vocals. I have often listened to remixes of popular song titles in which I'm sure the remix artist either extracted the vocals or obtained them in some way. I'm sure many original artists (especially the really popular ones) don't make an a cappella version of their songs. Thanks.