facebook code

https://code.facebook.com

Mobile:
Rebound: Android & JavaScript physics library

Web:
React: A JavaScript library for building user interfaces
Huxley: A visual regression testing framework

Data:
Presto: Distributed SQL Query Engine for Big Data
RocksDB: A High Performance Key-Value Store

Infra:
Wormhole: Moving data through space and time
HipHop VM: PHP server and virtual machine