3 free game console synthesis plugins. Emulations of some of the most iconic chips including the Ricoh A203 (Nintendo), PAPU (Gameboy), MOS Technology SID (Commodore 64, 128 & MAX) and TI SN76489 (Sega, BBC etc).
Cassette Drums is a freeware VST and AU plugin bundle from BRP consisting of three individual plugins: Cassette 606 VST, Cassette 808 VST and Cassette 909 VST.
Have a nice informative read about the Polish Radio Experimental Studio and download sounds from the 1960s-80s in Eastern Europe at Ableton.
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.