I've just uploaded a functional coroutine library for C, called libconcurrency. It's available under the LGPL. I think it's the most complete, flexible and simplest coroutine implementation I've seen, so hopefully it will find some use.
Next on the todo list are some more rigourous tests of the corner cases, and then extending libconcurrency to scale across CPUs. This will make it the C equivalent of Manticore for ML.
There is a rich opportunity for scalable concurrency in C. Of course, I only built this library to serve as a core component of a virtual machine I'm building, and that's all I'm going to say about that. ;-)