I believe there is a lot of research focused in this area right now. Besides Ripple, here is a list of other tech I happen to know of that is being worked on, including some notes about their current progress:

- - Proof of Stake: Still not mathematically proven enough to be acceptable. (Do note that there are multiple different variants of Proof of Stake that each work slightly differently)
- - Proof of Elapsed Time: Greatly hyped by HyperLedger, but it is proprietary technology by Intel that only works if you have a certain Intel chip, and places trust in the Intel company. I don't know enough about how this should work to know why it wouldn't be possible to e.g. make a fake hardware chip.
- - Proof of Burn. Not entirely sure how the progress is on this one.
- - The IOTA Tangle. Seems a very promising method to build a DAG rather than a chain. A problem of which I don't know how they want to solve it, is how it is prevented that new info is added as children of 'old' elements, essentially 'inserting something in the past'.
- - The Hashgraph. Currently closed source (the team does claim that it will be published as FOOS at some point though). Also builds a DAG in a different way than the Tangle does. It is easier to see how the network as a whole prevents 'insertions in the past'. What is not yet known, is how/or even if the Hashgraph can work in a public context; it currently requires a static number of nodes (where all nodes know each other at the start) and where a supermajority of nodes is online at all times. There are some vaguely-described extensions to this, but these do not explain how someone could join and leave at any time as people can do with other ledgers.

This list is by no means complete.


breathepublication.com/blockchain-just-became-obsolete-the-future-is-hashgraph-de4948609cbf