GNU Pth
GNU Portable Threading Library
http://www.gnu.org/software/pth/
"Pth is a very portable POSIX/ANSI-C based library for Unix platforms which provides non-preemptive priority-based scheduling for multiple threads of execution (aka ``multithreading'') inside event-driven applications."
GNU Pth is a Cooperative Threading implementation.
Sample application using GNU Portable Threads: http://www.rkeene.org/projects/info/resources/threads/test1-pth.c