PieceWork is a tweak to standard cryptocurrency PoWs that enables recycling of mining effort.
Resource-Efficient Mining (REM) repurposes innately useful workloads for mining effort. It relies on use of a trusted hardware technology called Intel SGX.
Snow White is the Proof-of-Stake system with rigorous security guarantees.
Interesting quote from that article:
Estimates vary, but it is likely that the Bitcoin network consumes roughly as much electricity as a nuclear reactor, about 1/3 of the entire electricity consumption of the entire country of Ireland.