E! Online has a list of celebrities tweeting their sadness and giving props to his trailblazing vision:

Snoop Dogg: “RIP MCA U are a Legend and a pioneer. #BeastieBoys4life”

Travis Barker: “Rest In Peace Adam Yauch /MCA. 4ever somebody i looked up to & had the pleasure of meeting. Damn this bums me out, we lost one of the best”

Kevin Smith: “RIP Adam Yauch. You made my teenage years more fun and your art made it easier for me to do what I do. For a Beastie, you sure were a beaut.”

Common: “God Bless the soul of Adam Yauch aka MCA of the Beastie Boys. RIP”

Rainn Wilson: “RIP Adam Yauch (MCA of the Beastie Boys). He was a super-genius, a visionary and, most importantly, a really good person.”

Rob Thomas: “holy s–t. R.I.P. ADAM YAUCH. MCA, you will be missed.”

Slash: “Sad day. MCA was f–king awesome. RIP man. We’ll miss you very much.”

Jon Cryer: “We are gonna miss all the greatness you still had left inside you. RIP Adam Yauch”

Mario Lopez: “Beastie Boys were such a part of my soundtrack as a youth growing up… R.I.P MCA…”

311: “Much love & respect, Adam Yauch. Our best to the #BeastieBoys family.”

Kal Penn: “Sad news about #MCA”

Milla Jovovich: “we lost one of our greatest talents today. my prayers and thoughts go out to the family and friends of Adam Yauch. Beastie Boys for life.”

Chris Hardwick: “RIP MCA. Interviewed him briefly once and he couldn’t have been sweeter or cooler. This sucks a lot.

Mandy Moore: “sending energy and light to Adam Yauch’s loved ones…..#RIPMCA”

Michelle Branch: “Just heard about Adam Yauch. Beyond bummed. So so so sad.”

Estelle: ” RIP Adam Yauch of the LEGENDARY Beastie Boys … damn.”

Josh Groban: “Oh no. This just sucks beyond measure. RIP MCA.”