Guild of Writers

I think this is as easy as it gets. Free and easy rarely go hand in hand.

And even with commercial programs you have to get the plugins (the actual sounds) separately.

It is a little confusing at first. Yes, Savihost is a VST plugin host. You put a copy of Savihost in the folder for each VST plugin, then you rename it the same as the dll file in that folder. Then you can open the renamed file and it's a virtual synth using the VST plugin. I know about VSThost, but Savihost is the one I use. It's much simpler.
Here's how it looks using one of my favorite vst synths:



Each vst synth is a virtual synth. I've downloaded maybe 100 of them over the years, but I have about ten or so favorites. I change the sound and make the synth do what I want and then I play the tune in my head and record it.

When I get writer's block or whatever there's a toy I play with get me back into making my own music. It's called Sounder. You simply open little windows in the program and play with the spiraling objects to make a tune.

If you are familiar with music notation, you may want to take a look at MuseScore (http://musescore.org/)
It is Open Source (therefore free) and if you install a good soundfont with it, you can produce fairly high quality music.