VirtualMIDISynth
coolsoft.altervista.org
coolsoft.altervista.org
And if anyone's interested in diving into MIDI, I took this class at CMU and it was incredible, all the slides and material are available online here: https://www.cs.cmu.edu/~music/cmsip/resources.html
Besides, a softsynth won’t sound good without a bit of reverb, and if you want to play, at some point you’ll want to record it.
Another point, however, is the software synth’s filesize - and indeed, GarageBand or Logic’s full instrument library can range from 5-90GB.
(Yes, you read that range right - depending on your choice of installation optionals, including loops and the recently-included ‘Alchemy’ library, you could be looking at 80GB).
On the negative side, GB didn't support MIDI-out. It was hobbled, like the budget version of Logic. But, then, there's always Logic (now at 1/5 of the price).
There's a video of her sitting on the floor in a million-dollar studio - using just Garageband on her Mac.
They're royalty-free for a reason.