Few mashup impress as much as Wait What's (WW's) work with Notorius B.I.G. and the XX. If you hadn't heard of either before hearing WW's masterpiece, you'd leave with love for the XXs' voices and Biggie's flow (three, if you count Wait What).
Wait What announced that the distribution of his mash-up was threatened by lawyers from one of those artists above. But that was some time ago, apparently, because his website offers the tracks - name your price and they're yours.