Joe Rogan, from the fear factor. He's not really groundbreaking, but hes an active proponent of psychadelics. He's a fan of DMT. YOu can listen to recordings of him talking about it on youtube.
Well, elover, I was gonna add Castaneda, The Grateful Dead (after all, that is mostly how L was spread in the US for a long time,) Paul Stamets and J.S. Chilton, but they're all on the list. Thanks for making posting no fun.
He wasn't really an activist or anything, but Syd Barrett did do a shit load of acid and was still excellent when he was with Pink Floyd.