There also some tools I think should be added: I'm not sure if linking to AddressSanitizer is okay (as its (an underused) part of Clang and GCC), but I consider include-what-you-use, unifdef, and maybe C++ Compiler Explorer a worthwhile addition.

What about mbedTLS (formerly PolarSSL)? It's dual licensed though.

https://code.google.com/p/address-sanitizer/(Apache 2.0)

https://code.google.com/p/include-what-you-use/ (same license as LLVM/Clang)

http://dotat.at/prog/unifdef/ (BSD)

http://gcc.godbolt.org/ (BSD)

https://tls.mbed.org/ (GPLv2)

Edit: midipix is something one should look out for. What it promises to support and how, sounds much more appealing to me than Cygwin or MinGW-w64, but it's still in early development: http://midipix.org/