Geonkick is a free software synthesizer that can synthesize elements of percussion. The most basic examples are: kick drums, snares, hit-hats, shakers, claps, sticks. It is released under that GPLv3 license.
Rekite is a small GUI toolkit. It is focused on enable to develop self-sufficient GUI plugins (for example, audio plugins) or standalone applications. Redkite is free software, released under GPLv3.