Ethereum
Blockchain with a built-in "world computer" — smart contracts via the Ethereum Virtual Machine. Vitalik Buterin, white paper 2013, mainnet July 2015.
Second largest cryptocurrency after Bitcoin. "The Merge" (September 2022) switched from Proof of Work to Proof of Stake → 99.95% less energy use. Smart contracts are usually written in Solidity (or Vyper). Layer 2s (Optimism, Arbitrum, Base) handle transaction volume. ERC-20 (tokens), ERC-721 (NFT), ERC-4337 (account abstraction) are important standards.