IOTA

IOTA is different in that it doesn't use a blockchain (with each block linked strictly in sequence), rather it has blocks that link forward to future blocks but that can skip blocks in between (the famous directed acyclic graph). Why is this important?

If you don't have to link to the last block that everyone agreed was minted, then you can have a more distributed ledger which can 'check in' vs requiring syncing all the time. This has the effect that you can have many times as many transactions at potentially extremely low transaction fees. This opens up a lot of opportunities for this technology that aren't feasable for traditional blockchains. 

The initial interest was in developing a blockchain for the Internet of Things space. Hence the name IOTA, but there is now wider industry interest to leverage this for other applications that need the properties of this blockchain.