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.