Cryptocurrency. If your first thought isn't "how could a scammer exploit this" you are doing it wrong.
I've thought about that for several hours now, and I'm sure scammers far prefer bitcoin. Folding@Home's lab director is a partner Andreessen Horowitz, so he has certainly had no lack of resources to defend against the possibility, and I am persuaded that the Indiana nonprofit behind FLDC is sincere and acting in good faith at present. If the Foundation is hesitant, they might sponsor an audit of either or both, but the Folding@Home project is so established that its article is featured on enwiki.
I have no financial interest in any cryptocurrency, and I never have, and I don't have a familial interest with anyone who I am aware has any either.
Best regards, Jim