The POCO C++ Libraries are powerful cross-platform C++ libraries for building network- and internet-based applications that run on desktop, server, mobile, IoT, and embedded systems.
JSON schema validator for JSON for Modern C++
EAMain provides a multi-platform entry point used for platforms that don't support console output, return codes and command-line arguments.
A small self-contained alternative to readline and libedit that supports UTF-8 and Windows and is BSD licensed.
Platform Abstraction Layer (both SCXCoreLib and SCXSystemLib) by microsoft.