Spectral, the first parametric Google font by Prototypo.
Discover Antique Gothic, Prototypo's latest parametric font.
Interactive Specimen of the parametric font Prototypo Elzevir
Parametric-Smart-Variable fonts using Prototypo
The company has been shut in 2020 but Prototypo code is open source and hosted on Github. Some users reported that an old version is still working on localhost: https://github.com/byte-foundry/prototypo/issues/767