Win32++ is a C++ library used to build windows applications. It is a simple, open source alternative to MFC. Win32++ supports the C++ compilers from GNU and Microsoft. It is an ideal starting point for anyone learning to program for windows, but also caters for the needs of professional developers.
WOSH is a multi-platform message-oriented middleware written in ANSI C++. Service oriented architecture, designed for network distributed computing. Already working: Audio multimedia, X10, remote control (WinMobile, GTalk) and much more..
The CoreLibrary is a multiplatform object library for C++ (Linux, Win32, MacOSX, PocketPC).
It provides many of the common objects found in Java (Strings, Threads, Collections, etc.) and has the ability to send objects across the network like CORBA.