"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.
It's a bit off-topic, but is there a reason to still use the original MinGW and not MinGW-w64? The list only mentions the original one.
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?
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.
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/
What are your thoughts on embedded scripting languages, for when your C needs a little more runtime flexibility? There are a few written in C and are designed to be slotted into a larger C program.
If you want to start a "computer vision" category, I've written a little library that might qualify: QuickBlob (LGPL connected component labeling)
I've worked with a lot of these (I work in game development, where this sort of thing is fairly popular), and IMO they're not worth the significant runtime cost and software complexity required to integrate them.
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.
Lots of goodies in there. I'll make a pitch for including some modern mallocs like jemalloc or tcmalloc in the list. Also the hdf5 library for numerical data storage is quite useful (pytables is built on top of it).
Which is not an amazing reason to get into uncanny valley territory with respect to GitHub's design and page layout - to the point of using GitHub's Octicons (which is entirely permitted by the license, for the record, but IMO contributes to an overall uncomfortable feeling when combined with other aspects).
"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.