Download the DSSI development kit and some DSSI plugins from the SourceForge project page.
The packages available are:
The dssi package contains:
jack-dssi-host) that can host any number of DSSI plugins with GUIs as a JACK application using the ALSA sequencer for MIDI input.
The FluidSynth-DSSI package contains FluidSynth-DSSI, a wrapper for the FluidSynth SoundFont-playing software synthesizer, allowing it to function as a DSSI plugin. (This was formerly part of the main DSSI package.)
The xsynth-dssi package contains the Xsynth-DSSI plugin, a classic-analog (VCOs-VCF-VCA) style software synthesizer with an editor GUI. Xsynth-DSSI was written by Sean Bolton, and was based on Steve Brooke's Xsynth code, but has since aquired polyphonic operation, band-limited oscillators, a better filter mode, and velocity-sensitive envelopes.
The dssi-vst package contains a wrapper plugin for Windows VSTs that enables them to be used by DSSI hosts running on Linux or similar on i386, using Wine. dssi-vst was written by Chris Cannam.