Simple music synthesizer app (like a toy-piano), but based on usual qwerty-keyboard for input.
Giada
Giada is an open source, minimalistic and hardcore music production tool. Designed for DJs, live performers and electronic musicians.
Rumpupuikko
Drumstick is a set C++ MIDI libraries using Qt5 objects, idioms and style. It contains a C++ wrapper around the ALSA library sequencer interface; ALSA sequencer provides software support for MIDI technology on Linux. A complementary library provides classes for SMF (Standard MIDI files: .MID/.KAR), Cakewalk (.WRK), and Overture (.OVE) file formats processing.
Geonkick
Geonkick is a synthesizer that can synthesize elements
of percussion.