The Roots made Wise Up Ghost with Elvis Costello. Elvis Costello famously 'sabotaged' the Beastie Boys' SNL performance. The Beastie Boys' "Paul's Boutique" was produced by Rick Rubin. Rick Rubin also produced Metallica's Death Magnetic. Metallica's Kirk Hammett performed in the Bonnaroo 2008 superjam with Les Claypool (and used to sell him pot in high school, apparently). Les Claypool performed on Big in Japan with Tom Waits.
Tyler the Creator is in OFWGKTA with Frank Ocean. Frank Ocean performed on John Mayer's album Paradise Valley. John Mayer has worked with producer Steve Jordan. Steve Jordan played drums on on Bob Dylan's Down in the Groove. Bob Dylan performed with Wilco on the Americanarama tour. Jeff Tweedy, John Stirratt, Glenn Kotche, and Pat Sansone played on The Sun Came Out with Phil Selway and Ed O'Brien.