Mash ups are usually pretty silly, but every so often someone puts something genuinely worthwhile together. The Wugazi album 13 Chambers, where producers Cecil Otter and Swiss Andy smooshed Fugazi and Wu Tang together is a good time, and The Grey Album, where Dangermouse melded Jay-Z's Black Album onto The Beatles' White Album, is also fun.
Here's a new one though: Producer ToToM has combined a number of tracks by rapper El-P of Run The Jewels and Company Flow and vocal performances from Queens Of The Stone Age frontman Josh Homme, and compiled them together in a free-to-stream album called El Q. It's worth a listen: