Neo is an open-source community-driven blockchain platform. The main products you can find here are Neo blockchain node, developer tools and documentation. PRs and issues are welcome in every repository.
You can get the node from the neo repository. Take a look at neo-modules as well, because it contains many useful plugins for it (like RPC server that is not a part of the default node).
To develop smart contracts for Neo in C# you need neo-devpack-dotnet. neo-debugger can be used to debug contracts written in any language. See also:
proposals is where Neo standards (aka Neo Enhancement Proposals or NEPs) are maintained. These are the ultimate reference for protocol operation and inter-component interfaces. They're however a bit more formal and we recommend starting from documentation below for beginners.
docs is what drives docs.neo.org, the reference source of documentation on concepts and system APIs. neo-dev-portal is the repository behind developers.neo.org which covers more diverse spectrum of tools and provides more complete integration examples.
neo.org is about neo.org contents, if you've spotted a typo there you're welcome to submit issue/PR.
non-native-contracts contains some well-known contracts (like NNS) deployed to both testnet and mainnet and useful for general audience.