LMMS is available for free with source code licensed under GPL and available on GitHub allowing anyone to edit and extend the software as they see fit.
Not only it has various type of synths like standard wave form, fm, additive synth, etc. But it also has modules that able to detect the amplitude, midi pitch and velocity in real time and convert into automation signal for making some advance sounds with no VST need.
Particularly looks fuzzy on large displays. Perhaps it needs to be built against a newer GUI toolkit, I've seen the problems like this resolved for Notepad++ and SQL Server Management Studio. Perhaps this is principally a font issue?
Runs on almost any platform that SDL supports, and has been successfully built for Windows, Mac OS X, Linux, FreeBSD, OpenBSD, AmigaOS, BeOS, and even the Wii.
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