Additive synthesis is a method of sound creation based on adding sine waves of different frequencies to create more complex sounds. Each sine wave typically changes amplitude over time to create evolving sounds.
The easiest sounds to create using additive synthesis are organs and basses. A drawbar organ works using a limited form of additive synthesis, with each drawbar representing an additive frequency.
The first synthesizer was an additive tonewheel-based organ called the Telharmonium, created in 1897.
This list excludes tonewheel-based organs, even though they are also additive synthesizers.
In addition to hardware implementations, many VST plugins implement additive synthesis.