The newmat matrix library (Robert B Davies). This library is only used for matrix SVD decomposition and inversion in Least Squares methods (Least Squares method was written only for tests purposes... Don't expect anything from it...).
The atominfo library(Ralf Grosse-Kunstleve). This library is used to determine the atomic scattering factors for X-Ray, neutrons, as well as anomalous scattering factors and the atomic number.
The SgLite library(Ralf Grosse-Kunstleve). Note that this package is part of the PyMOL Molecular Graphics System (used with permission), and is not free software. It is used to derive symmetry operations from a given spacegroup symbol or number.
The Blitz++ array library (Todd Veldhuizen). This library is used for data storage and calculation in arrays and vectors. (Currently it is not used because of the huge memory requirements when compiling blitz++ expressions using the gcc compiler. Instead the CrystVector and CrystMatrix are used, emulating the blitz interface, but without the smart handling of mathematical expressions.) Support for the Blitz++ should come back some time...