It seems like the most prolific things are hallucinogens on one side and heroin on the other.
Hallucinogens have a particular bright side, it seems like those who have been able to garner creativity from it remember exactly how it felt, it does permanently change the way you feel and see the world (not from a brain damage perspective - more like learning multiplication or division permanently changes the way you see numbers, a learned add-on perspective). It can be very dangerous to some. My own testimony - music was already intimate with me, it became that much more intimate and I even dialed in more on what I had at the bottom of myself, what I really wished to get out. Its like taking everything from your subconscious to a very epic/religious kind of level and, so long as you aren't getting way out of hand and eating ten-strips, its a lucid enough experience not to lose the meaning or relevance of the next day or the day after - as its a very distinct sort of feeling that one can bring back to mind or ponder without needing anything.
Heroin - I have no idea, I'm already well past my try things stage, and its obviously even more something never to be recommended. Still, a lot of the deepest, most dynamic, most prolific singers or front men/organizers of bands from the late 80's through 90's were H-bombs, enough even died from it. Alice In Chains, Skinny Puppy, Pantera, Sublime, Red Hot Chili Peppers, that's only to count a small handful who've been known for it or who had band members die or nearly die. I would like to think that Mainard from Tool never dabbled in that stuff, if so he's evidence that there can be ultra-deep bands out there that don't dabble in smack. Then again I have to wonder if the reason a lot of creative types in the US go out that way is simply because, the way our culture is, its the only way that they found the ability to dive that deep into themselves (the culture obviously doesn't lend itself to nurturing emotional or psychological depth, more often punishes it).
Of course people will say that there's also something to baking and sinking in to a deep/mood-intensive tune. That effect as well as the first is just another thing where, if someone's even done it a few times, they know its there and they know what's good without the need to keep going back into that state.