Vitalik's annotated eth2 spec. Not intended to be "the" annotated spec; other documents like Ben Edgington's https://benjaminion.xyz/eth2-annotated-spec/ also exist. This one is intended to focus more on design rationale.
Ethereum peer-to-peer networking specifications
Specification for the Execution Layer. Tracking network upgrades.
Guidelines and training material to write secure smart contracts
Collection of RESTful APIs provided by Ethereum Beacon nodes
Official repository for specifications for the Portal Network
Upgradeable proxy with the least possible gas overhead.
Collection of APIs provided by Ethereum execution layer clients
Ethereum Proof-of-Stake Consensus Specifications
Agent-based simulation environment for PoS Ethereum
Specification for the external block builders.
Common tests for the Ethereum proof-of-stake consensus layer
Informative spec for beacon node metrics
A reference implementation for the Wormhole blockchain interoperability protocol.