NAVIPEDIA GNSS tools can be accessed from here.
RTKLIB is an open source program package for GNSS positioning. Please visit the RTKLIB website for more information.
GPSTk provides a C++ open source library and a suite of applications for GPS processing problems. Please visit the GPSTk website for more information.
GAMIT, GLOBK and TRACK form a comprehensive suite of programs for analyzing GNSS measurements primarily to study crustal deformation. Please check the following link for further information.
GNSS-Inferred Positioning System and Orbit Anlysis Simulation Software. More details can be found here.