Microsoft taps open source LLVM compiler for cross-platform .Net 

Not all open-source licenses are the same.

 .NET crypto done right. Free, open source (MIT license). 

Open Source .net libraries that make your life easier 

Docker is a popular open source containerization tool.  

A free, Open Source, Cross Platform C/C++ IDE 

 PostCSS is a tool for transforming CSS with JS plugins. These plugins can support variables and mixins, transpile future CSS syntax, inline images, and more.
PostCSS is used by Google, Twitter, Alibaba, and Shopify.
PostCSS itself is very small. It includes only a CSS parser, a CSS node tree API, a source map generator, and a node tree stringifier