The Swiss Ephemeris is the high precision ephemeris developed by Astrodienst, largely based upon the DExxx ephemerides from NASA's JPL. The original release in 1997 was based on the DE405/406 ephemeris. Since release 2.00 in February 2014, it is based on the DE431 ephemeris released by JPL in September 2013. The source can also be retrieved here.

They have two license options - one, the GPL, which only means that any code you write must also be GPL. And second, their license, which is paid.