The SAGA C++ Reference Implementation is a set of free cross-platform libraries written in C++ and Python which provide a set of high-level interfaces and runtime components that allow the development of distributed computing and grid computing applications, frameworks and tools. SAGA is the first complete implementation of the Open Grid Forum Simple API for Grid Applications standard GFD-R-P.90. SAGA is available for all major operating systems, including Linux and other Unix-like systems, Microsoft Windows and Mac OS X. SAGA is open source and licensed under the Boost Software License. SAGA can be used to develop scalable and portable large-scale distributed applications, frameworks and tools.

More info, latest version & other good stuff: http://www.saga-project.org

Quick links to API packages: