Add64 is a realtime additive softsynth for linux, developed by Matthias Nagorni. The GUI interface departs from the traditional knobs-and-buttons approach by using a spectral graph to permit the oscillators and related parameters to be modified. The screenshot above shows the harmonics window for Spectral Oscillator 1. There are two "scenes" and each scene has 3 oscillators and a noise parameter. To run Add64, just open a terminal and type
Add64. It will ask you to choose the polyphony, the default setting is 4. (NB: Due to high CPU usage, there are some audible glitches in the new demos above (which occurred only during the recording process, not during normal performance). A good manual is included in the tarball at Sourceforge (see link below).
A few sounds to begin with: Add64 Patches 01
Some new presets: Add64 Patches 02
Add64 download at Sourceforge