GSL - GNU Scientific Library
The GNU Scientific Library (GSL) is a numerical library for C and C++ programmers. It is free software under the GNU General Public License.
The library provides a wide range of mathematical routines such as random number generators, special functions and least-squares fitting. There are over 1000 functions in total with an extensive test suite.
The current version is GSL-1.9. It was released on 21 February 2007. This is a stable release.
The complete range of subject areas covered by the library includes,
Complex Numbers Roots of Polynomials Special Functions Vectors and Matrices Permutations Sorting BLAS Support Linear Algebra Eigensystems Fast Fourier Transforms Quadrature Random Numbers Quasi-Random Sequences Random Distributions Statistics Histograms N-Tuples Monte Carlo Integration Simulated Annealing Differential Equations Interpolation Numerical Differentiation Chebyshev Approximation Series Acceleration Discrete Hankel Transforms Root-Finding Minimization Least-Squares Fitting Physical Constants IEEE Floating-Point Discrete Wavelet Transforms
Unlike the licenses of proprietary numerical libraries the license of GSL does not restrict scientific cooperation. It allows you to share your programs freely with others.