GSS (Grus Sparse Solver) is a very good sparse matrix solver, but is not free, it is the spd version of the GSS, that is, symmetric positive definite sparse matrix solver, is open source, the authors claim that can be solved within 1 minute on the one million unknowns equations.
File list:
gss_6_demo.c
gss_6_demo.for
gss_6_demo.h
GSS_DLL_6D.dll
GSS_DLL_6D.lib
GSS_SPD PERSONAL, NON-COMMERCIAL USE LICENSE.txt
GSS_spd quick start user guide.pdf