Awesome-C: A curated list of awesome C frameworks, libraries, resources
notabug.org
notabug.org
"Venerable" would be a much more gracious way to describe the compiler that brought us so much, to say nothing about the fact that it's still actively maintained today.
I also didn't know that GNU refers to the MIT license as Expat license.
The list also links to several PDF versions of books that are still sold and also not published under a free license. Just mentioning them should be fine. We all know how to use an online search engine if we want to get a copy, don't we?
As for the PDF links - good point. I'll amend that too.
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/
If you want to start a "computer vision" category, I've written a little library that might qualify: QuickBlob (LGPL connected component labeling)
In my experience, it's almost never that much harder to develop the code in C or C++ (especially given how much better C/C++ debuggers are from those found in scripting languages).
Most of the dynamism and flexibility that actually give you benefits (IMO module systems and hot code reloading -- the latter of which is actually easier to do in C than in a lot of other languages) can be achieved using dynamically linked libraries, anyway.