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 (ex. rosegarden).