[Z]

They seem to be used to either introduce a new artist (i.e. established act introduces a new one) or a new artist features an established artist, or it's a collaboration between similar artists or totally different ones. I think I've covered all the bases there. I think the only kind that I really appreciate are two different types of musician working together, e.g. Linkin Park/Jay Z (despite someone up the thread saying they disliked it.) One of my favourite recent collaborations has been Keane ft. Tigarah - a Japanese female emcee. Bizarre mixes often create really interesting music. I dislike the current trend of grunge/rap artist(s) featuring pop acts (or vice versa) because they all sound really same-y. That said, I do have a soft spot for Tinchy Stryder/Amelle, Chipmunk/Esmee Denters and most recently T.I./Christina Aguilera... so yeah.