The General Astrodynamics Library (GAL) 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 astrodynamical routines such as
ephemerides, earth orientation, and orbit propagation. There are over 170
functions in total with an extensive test suit

WWW: http://homepage.mac.com/pclwillmott/GAL/