Yoshimi is a software audio synthesizer, originally forked from ZynAddSubFX. Follow the links in the header for help with downloading and running it. You can also check out the manuals for detailed help, and listen to examples of music created with Yoshimi. Once you have it all set up, here are just some of the features you will find:

• Three separate engines, each with distinctive characteristics, can be used together or individually
• Sixteen engine kits for the ultimate in multi-layering of engines, or for synthesized drum kits
• Vector Control, manage up to four independent sounds on any channel with faders or an X/Y midi joystick
• Solo step through up to 16 sounds seamlessly while playing in real time (including vector sounds)
• Microtonal Tuning, Scala-compatible scales and keymaps
• Extensive Midi-Learn, capture almost all internal controls and many switches with ordinary 7-bit values, or NRPNs, or even 7-bit NRPNs, then edit the responses
• Command Line Access, headless operation and high accessibility, for those with sight- or motor control issues
• Rich variations of tone and timbre, Yoshimi provides the means to ensure no two notes are ever identical – just like in the real world

yoshimi.sourceforge.net
en.wikipedia.org/wiki/Yoshimi_(synthesizer)