Substrate.dev - Starting point for learning about Substrate, which is a Rust-based framework for developing blockchains. Moonbeam is developed using Substrate and uses many of the modules that come with Substrate.
Polkadot.network - Learn about Polkadot, the vision behind the network, how the system works including staking, governance, etc.
Polkadot-JS Apps - A web-based interface for interacting with substrate based nodes including Moonbeam.
Solidity Docs - Solidity is the main smart contract programming language supported by Ethereum and Moonbeam. The Solidity docs site is very comprehensive.
Remix - Web-based IDE for Solidity smart contract development that is compatible with Moonbeam.
Truffle - Development tools for Solidity, including debugging, testing, and automated deployment that is compatible with Moonbeam.