The other day, Neal Brennan (co-creator of Chappelle's Show) tweeted a link to a YouTube video he uploaded. Apparently, the Beastie Boys were supposed to be one of the musical acts to appear during the third season of the show. They recorded their song and then Chappelle left, so Brennan decided to upload the footage in honor of MCA. Viacom took it down, and every other link I've found was down, too.
But I finally found one that is working. I don't know how much longer the video will be up, but you should definitely check this out while you can if you are a Beastie Boys fan. Always cool to get a glimpse at never before seen footage.
http://hypervocal.tumblr.com/post/22648835036/unaired-beastie-boys-video-from-chappelles