Jeskola Buzz is a free modular software music studio environment centered around a modular plugin-based machine view and a multiple pattern sequencer tracker. It´s been around since 2000 and includes lots of plugins. To see/listen what is possible with it, here´s a full production session with it by Roman Buehler:
Frequency modulation synthesis (or FM synthesis) is a form of sound synthesis where the frequency of a waveform, called the carrier, is changed by modulating its frequency with a modulator. Most famously, the Yamaha DX7 synth features this type of synthesis. Learn how to sound design with this method and check out free alternative synth plugins.