Valhalla Tracker aims to be a MIDI sequencing companion for GNU/Linux audio setups. Adhering to Unix philosophy, it tries to do one thing well

• input/output relies 100% on JACK
• which allows for sample exact timing
• and freewheel rendering
• fits well between a MIDI controller and Carla/fluidsynth/Zyn
• fast workflow with lots of keyboard shortcuts
• allows for "unheard of" time signatures
• starts in one second, builds in five
• fits on a floppy

The sequencing engine is written in C, has a Python library on top of it, which can be used headless (for generative composition or whatever). The GUI is also written in Python and uses GTK through GObject Introspection.