Overtonium is a free, open-source additive synthesizer from Benjamin Dehli (Dehli Musikk), released in September 2026. It is laid out like a 32-channel mixer, where each channel is a sine oscillator locked to a harmonic of the played note, with its own tuning, envelope, modulation and stereo placement, plus a separate noise channel.
Its signature feature is a TUNE control that sweeps every partial continuously between equal temperament and pure just intonation, backed by six historical keyboard temperaments and an inharmonic stretch. It runs as VST3, AU, LV2 and standalone on macOS, Windows and Linux, and it is free under the AGPLv3 license. For the full breakdown, see the Overtonium first look.