if you mean you just want to make music and arent too bothered about recording then Cardinal is a free modular eurorack synth you could mess around with
or you use a VST host like Kushview's Element and load up on all the free VST instruments and effects that are out there. you would just need a midi keyboard hooked to it then or use Cardinal to generate note patterns
or something like Orca if you want to go off the deep end!