Blockchain
*Lots of hype, tarnished by crypto-libertarianism and environmental impacts of e.g. Bitcoin, but interesting technology nonetheless. Distributed ledgers of one sort or another could be used for beneficial purposes (see e.g. DisCOs).
Both blockchain and digital currencies offer a great deal of potential, but Bitcoin itself is a deflationary, capitalist medium that functions primarily as an investment asset to concentrate wealth, and is mostly favored by right-libertarian hard money ideologues.
– Center for a Stateless Society » Review: A DisCO Manifesto
However, the really interesting point about Bitcoin is not the currency itself, but rather the fact that its protocol – the blockchain – can be used to implement many other applications which have nothing to do with money.
Blockchain's one way of implementing digital cash.
1. Environmental impact
If you understand the difference between Proof of Work and Proof of Stake, then you should see that PoS blockchains don't have the horrendous problem that PoW do. Sure, they consume some energy, just like any other computing. But not on principle, as with PoW.
The 15 Most Sustainable Cryptocurrencies for 2021 - LeafScore