libsharp
Library for spherical harmonic transforms (SHT)
...Libsharp is a collection of algorithms for efficient conversion between maps on the sphere and their spherical harmonic coefficients. It supports a wide range of pixelisations (including HEALPix, GLESP, and ECP). This library is a successor of libpsht (http://sourceforge.net/projects/libpsht). A scientific overview over the functionality is given in the paper available at http://arxiv.org/abs/1303.4945.