hexter is a software synthesizer that models the sound generation of a Yamaha 
DX7 synthesizer. It can easily load most DX7 patch bank files, accept patch 
editing commands via MIDI sys-ex messages (ALSA systems only), and recreate 
the sound of the DX7 with greater accuracy than any previous open-source 
emulation (that the author is aware of....).

hexter operates as a plugin for the Disposable Soft Synth Interface (DSSI);
you need a program to host it (e.g. rosegarden).

This requires dssi and liblo.