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.
Free Kontakt version of the Concertmate 300 (You might remember it from Casio or Tandy) for some Chiptune sounds. Get it from Sonus Instruments.
Free version of the Alpha VST synth. Basic interface, good sound quality though. Download from KVR.