If you ignore the credit rating system, and replace its hash algorithm with variable-length (expanding) one, that's basically what Bitcoin is. (Inversion of variable-length collision-resistant hash functions is NP-hard. I had to ask on that one.)

[EDIT: That question has been dead for a while, but now that I posted a link, it got another answer which basically repeats the first answer and needlessly retreads why I had to rephrase the question in a way such that the hash inversion problem has a variable size so that asymptotic difficulty becomes meaningful, thus being non-responsive to the question as now phrased. I hope it wasn't someone from here that clicked that link.]

They've made a lot of progress getting games to derive protein-folding results, but I think there's a lot of room for improvement there (better fidelity to the laws of the protein folding environment so players can develop an "intuition" of what shapes work, semiotics that are more suggestive of the dynamics of their subsystems, etc).