WildMIDI is a simple software midi player which has a core softsynth library that can be use with other applications.
Currently there is a small command line player that accompanies the library which gives just one example of the use of the WildMIDI library. The next release will also contain a "self contained" player which will play midi files without the extra overhead the that occurs with the library. This is achieved by having a core source that is shared between the library and "self contained" player.
The gus patch set (for example from Timidity which you can find in PETget package manager)