Several codenamed prototypes of the Ethereum platform were developed by the Foundation, as part of their Proof-of-Concept series, prior to the official launch of the Frontier network. "Olympic" was the last of these prototypes, and public beta pre-release. The Olympic network provided users with a bug bounty of 25,000 ether for stress testing the limits of the Ethereum blockchain. "Frontier" marked the tentative experimental release of the Ethereum platform in July 2015.
"Homestead" was the first to be considered stable. It includes improvements to transaction processing, gas pricing, and security.l upgrades, which are important changes affecting the underlying functionality and/or incentive structures of the platform.
There are at least two other protocol upgrades planned in the future: "Metropolis" is intended to reduce the complexity of the EVM and provide more flexibility for smart contract developers. Metropolis also adds supports for zk-SNARKs (from Zcash); with the first zk-SNARK transaction occurring on testnet on September 19, 2017."Serenity" should include a fundamental change to Ethereum's consensus algorithm to enable a basic transition from hardware mining (proof-of-work) to virtual mining (proof-of-stake). Improvements to scalability, specifically sharding, are also said to be a key objective on the development roadmap.