PTLib (Portable Tools Library) is a moderately large class library that
has it's genesis many years ago as PWLib (portable Windows Library), a
method to product applications to run on both Microsoft Windows and Unix
systems. It has also been ported to other systems such as Mac OSX, VxWorks
and other embedded systems.
It is supplied mainly to support the OPAL project, but that shouldn't stop
you from using it in whatever project you have in mind if you so desire.