Java Simple Sampler allows you to play sampled on Midi events. We are at this point (july the 13th 2008) at a very early release. The code is under GPL v3 licence.
You can access the code through cvs only for now :
cvs -d:pserver:anonymous@jssampler.cvs.sourceforge.net:/cvsroot/jssampler login
cvs -z3 -d:pserver:anonymous@jssampler.cvs.sourceforge.net:/cvsroot/jssampler co -P JavaSimpleSampler