Bitcoin's extreme blockchain congestion, coupled with high transaction fees, of the last week has shown us that it a solution to the scaling problem is needed.
There are, however, some altcoins that already attempt to solve it and have, in part, some interesting solutions to offer. Some of them are still in the testing stage, others are already available.
The list is still a draft and not complete! Everybody that thinks (s)he knows an altcoin that also does solve that problem can post his/her "candidates" here. I will review it and if I think it adds something interesting, I'll add it. I'll only add coins that have at least a testnet running and a estimated release date. If you spot an error, I'm open for feedback.
Already running:Cryptonite (XCN)What they solve: IBD, Storage
How they solve it: Mini-Blockchain
Bitcointalk threadCryptonite is already running since 2014. It combines Bitcoin's code with an account tree, and periodically (approximately once per week) blocks are "pruned" from the chain while the account tree is updated. Only a "proof chain" with the block headers is conserved. The result: a very fast-syncing (~1 hour) and very small blockchain which almost doesn't grow.
Downsides: They had a critical flaw in the code until September 2017, but it has been fixed.
IOTAWhat they solve: IBD, Storage, Latency
Hoy they solve it: Tangle (DAG) snapshots
Bitcointalk threadIOTA is one of the most experimental and different cryptocurrency projects. Apart from their well-known usage of a DAG instead of a blockchain, which drastically reduces latency between nodes, they offer also a scaling solution: Once a transaction has been stabilized, it can be "forgotten" by the network and instead the state of the affected accounts is saved on a snapshot. So the general way they deal with the problem is pretty similar to XCN's.
Downsides: IOTA still uses a centralized node, the "Coordinator", that is part of the validation process. However, the IOTA developers plan to shut it down once the network has matured.
ARDORWhat they solve: IBD, Storage
How they solve it: Child chain snapshots
Bitcointalk threadARDOR is the second generation of the NXT project. It carries on a novelty: Almost all transactions are not recorded on the main chain, but on "child chains". These are a bit like sidechains, but they temporarily (for 1440 blocks) become validated and stored by the main chain - after this timeframe they are pruned. Child chains can be stored by archival nodes if it's needed.
Downsides: Not fully open source (JPL)
Lisk (LSK) (originally Crypti)What they solve: IBD, Storage
How they solve it: Sidechains
Bitcointalk threadLisk is the only coin I have on my radar that provides a mechanism for pegged sidechains. Pegged sidechains are also planned for Bitcoin, but only centralized ones (e.g. Blockstream Elements) have been delivered until today. Lisk's solution, however, is also not totally decentralized; the sidechains are issued by an entity that must care for the security of the chain via Delegated Proof of Stake (DPOS). But it's a step in the right direction, because the peg itself seems to be managed by an internal mechanism and not by the issuer.
Downsides: The mechanism is not totally decentralized.
Some other interesting future projects (without release date)- Lightning Network (testnet already working) / Raiden network
- Ethereum's Plasma child-chain architecture (planned) and "sharding" (planned)
- Drivechains
- IOT Chain
"What they solve" section- IBD: Initial Blockchain Download. That means that new
full nodes must download less data to participate. "Light" (SPV) nodes do not count as almost all cryptocurrencies offer them.
- Storage: Full nodes have to store less data on their hard disk.
- Latency: There is less latency between nodes that transfer transactions or blocks.