Nothing else does what MIXMIDI does. It doesn't just pile tracks one on top of another. It reconciles the tracks of one file with the tracks of one or more other files, producing a result that is the average of the songs--not a bunch of overlapping notes.
MIXMIDI is available only as FORTRAN source code for Linux. It could readily be ported to other operating systems, if there is enough interest.
Public domain. It may be used freely by any person for any purpose, pleasure or profit.