The LinuxThreads library implements Posix 1003.1c threads on (as the name reveals) Linux. It provides kernel-level threads where scheduling is handled by the Linux kernel. Thus, unlike user-level thread packages, it can take advantage of multiprocessors.

This C library implements POSIX threads on a variety of operating systems, including Linux, FreeBSD, SCO Unix, Solaris, SunOS and DOS.

(Since the original QpThread does not appear to be available any more, I'm listing this fork of QpThread.) The QpThread is a C++ framework for threads and Unix signals that operates on top of POSIX threads. It supports deferred cancellation (through C++ exceptions), timers, thread pools, etc.