Use Gnu Scientific Library as if it were writtem in C++.
The ccgsl provides simple C++ wrappers for the GNU Scientific Library. It uses Java-like shared-pointer classes in place of structs to avoid direct memory allocation/freeing and to work better with the STL. It lets you construct functions for optimisation, root-finding and the like from C++ member functions, making it easier to integrate with existing C++ code. It also provides C++ exceptions.
a free cross platform calculator for calculating BIG numbers
BigAl is a free open source and platform independent software for calculating really BIG numbers. It supports not only standard arithmetic, but also exact period determination, predefined factorials, fibonacci with seeds, factorization, nth root, etc