I think it’s cool when an artist helps out another up and coming (Or established) artist by being on their track without needing to be noticed.
I’m sure they get some notoriety in the end, maybe on the album cover or in the booklet.
But I found an article that reveals 23 songs that you might not know had secret artists on them.
Lenny Kravitz has helped out Madonna.
And Ke$ha has helped out Paris Hilton at one point!