Portable thread library for C++

TinyThread++

Need portable threads for your C++ app? Is C++11 unavailable for your target compiler(s)? Is Boost too large?

Then you need TinyThread++!

TinyThread++ implements a fairly compatible subset of the C++11 thread management classes.

Open source!
Very portable (it has been tested under Windows, Mac OS X, Linux, OpenSolaris and FreeBSD, and should work under most POSIX compatible systems).
Fairly faithful to the C++11 standard (see the January 2012 draft: N3337, chapter 30).
Minimalistic (no external dependencies, and easy to include in a project).
Minimal overhead — most functions generate compact inline code.
As a bonus, the library includes a very fast user space mutex class (fast_mutex).

The latest release is v1.1 (2012.05.07):

The Doxygen generated documentation can be found here.

The library is released under the zlib/libpng license, which makes it perfectly free to use for any application (including commercial applications).

If TinyThread++ is not what you need, you may try:

just::thread - a complete C++11 thread library implementation (commercial).
Boost C++ libraries - contains threading classes (not 100% compatible with C++11).
POCO C++ Libraries - contains threading classes (not really C++11 style, but similar functionality).
TinyCThread - a C alternative.