Meta

1. React: A JavaScript library for building user interfaces, popular for its component-based architecture and virtual DOM. It's widely used in both Facebook and Meta's applications, as well as numerous third-party web applications.
2. GraphQL: An open-source data query language developed by Facebook. GraphQL provides an efficient alternative to REST APIs, allowing clients to request only the necessary data, reducing over-fetching or under-fetching issues.
3. Flow (Static Type Checkers): A static type checker for JavaScript that helps find errors in code early on through type annotations. It's used extensively at Meta to improve code quality and maintainability.
4. Hack: A programming language developed by Facebook for HHVM (HipHop Virtual Machine). Hack is a typed language compatible with PHP, designed to help developers write cleaner, more reliable code.
5. HHVM (HipHop Virtual Machine): An open-source virtual machine designed to execute programs written in PHP and Hack. It was initially created by Facebook to improve performance and reduce resource usage for running their massive codebase.
6. OSQuery: A tool that allows you to query operating systems like a database, enabling real-time insight into what's happening on your infrastructure. It was originally developed for monitoring internal Facebook infrastructure but is now widely used in various industries.
7. Phabricator: A suite of web applications for software development, including source code management, code review, and bug tracking. Meta still uses it internally, demonstrating its continued maintenance and relevance.
8. RocksDB: An embeddable, persistent key-value store developed by Facebook for fast storage. It's optimized for fast, low-latency access using flash storage and is used in many production environments beyond Meta.
9. Pants (Build Tool): A build system that helps manage large codebases with speed and correctness. While not as well known as others on this list, it's crucial for maintaining the scalability of Meta’s codebase.
10. Presto: An open-source distributed SQL query engine designed to process large datasets. Initially developed by Facebook, Presto is now a top-level project at the Apache Software Foundation and is used extensively in big data analytics across many companies.
11. TensorFlow (Early Contributor): Although not solely created by Meta, it's worth mentioning that Facebook has been an active contributor to TensorFlow, Google's open-source machine learning framework. This collaboration reflects the importance of AI and ML within Meta's operations.
12. Pyro: A research framework for probabilistic programming built on PyTorch. Although no longer maintained actively by Meta, Pyro was influential in pushing forward probabilistic AI research.
13. Rosetta: An open-source code translation tool that enables developers to write code in one language and automatically convert it into another. It's used within Facebook to maintain consistency across different programming languages and to ease the transition between them.
14. Sapling SCM: A privacy-preserving cryptocurrency protocol developed by Meta (then Facebook) for its Novi wallet, designed to facilitate fast and private transactions on the Diem blockchain.