Paraphrasing the website:

  The ZThread package is an advanced object-oriented, cross-platform, C++
  threading and synchronization library.  It provides a high level
  abstraction of the native threading mechanisms to offer a great deal of
  flexibility and control.

  ZThread features interruptible Thread and Runnable objects for C++, control
  objects and implementations of concurrency design patterns.  This includes
  semaphores, mutexes, condition variables as well as other more complex
  components.

ZThread implementations are provided for POSIX and Win32 platforms.

WWW: http://zthread.sourceforge.net/