MIPCL (http://www.mipcl-cpp.appspot.com/) is a very fast and free (as beer, but not open source) solver for (Mixed-)Integer Linear Programs ((M)ILPs).

EDIT: In Hans Mittelman's benchmark of MILP solvers (http://plato.asu.edu/bench.html), MIPCL is very performant: http://plato.asu.edu/ftp/milp.html