- "Layer 2" networks: Most transaction volume happens off-chain. POW blockchain remains as a settlement layer. Quite a few proposed protocols. Lightning network  is the most well known. First mainnet lightning transactions were sent in December 
- Sharding: Split the blockchain into lots of different blockchains with a subset of miners validating transactions on each sub-chain. Miners are randomly assigned a new sub-chain on a regular basis to stop collusion. Each sub-chain has it's own address space. Sub-chains can communicate using asynchronus message passing. Ethereum has a draft specification  and is starting to implement some test clients .