All the code I write is C99. What counts as a "large project" is open to interpretation though.

I sure have.

I work on embedded software for industrial assembly tools.

I wrote 14.62 hours worth of C89 yesterday, and 2.33 hours so far today. For PyParallel. (https://bitbucket.org/tpn/pyparallel)

Various bits of numerical analysis. The overall framework and plumbing is written in Python, as are all the algorithms initially. Then for those that are too slow, or need to be run lots of times, algorithms are reimplemented in C as a Python extensions.

The existing Python implementation then helps with debugging and testing.

The Linux kernel uses various C language features, including both C99 (such as designated initializers) and GCC extensions.

I have. The project is an event based http server and client. Mostly I work with libevent, redis, and openssl. We don't build our software on Windows so we don't have to worry about the limitations on that platform (non-POSIX mostly).

All my code is C99. Computational chemistry program that ships on Linux, Mac, and Windows.

what do you compile with for windows?

You can use GCC and Clang directly on Windows, both implement 100% C99. Alternatively, Visual Studio 2013 implements most of the C99 standard (not a complete implementation though).

GCC: only as Mingw or MSYS? You can use intel too, I wanted to know what this person was using, not what options there were.

You don't need to install MinGW or MSYS in order to use GCC (I assume you are interested only in C99 here and not in POSIX programming). Here is an example of a standalone GCC (contains C, C++ and Fortran compilers):

http://www.equation.com/servlet/equation.cmd?fa=fortran

I'm currently writing a game in C99.

Compilation times, no need for inheritance and the fact that most libraries I'm using are written in C were the reasons I chose C over C++ or something else.

I have, and most of the linux libraries that I work with are C.

Yes, I have and I found this was useful. Would like more sites like this or a central location for standard tricks which are tagged as being C99 or GNU specific.

I'm working on a music player client (Currently only terminal based, but that could change).

I write C code (not always C99) for automotive/racing.

I was working on an assembler/linker in C last night.

SIP phone software. Display and self-test code today.

pngquant is C99 https://github.com/pornel/pngquant

I have also. DSP code in an embedded environment.

Me; a high-security network crypto device.

Everyday for a router operating system.

Waves hand. Multiple OS libraries.

Here's one.

An MPI library.