atomic_ops
atomic_ops is a library of portable primitives for building lockfree multithreaded programs in C, by Hans Boehm of libgc fame. The basic atomic operations portion of the library is MIT-licensed, although it also includes a GPLed addon library which provides nearly lockfree stacks and a lockfree malloc implementation.
Update: Hans Boehm wrote to say that the older distributions are to be avoided in favor of the copy of the library in the libgc tree, which he has been keeping more up-to-date. It is available from libgc CVS at:
:pserver:anonymous@bdwgc.cvs.sourceforge.net:/cvsroot/bdwgc/libatomic_ops-1.2
libatomic_ops also appears to be available in many Debian-based distributions
as libatomic-ops-dev, in a version based on the last official release but
with a lot of additional patches.