In 2009 I joined the Hydrogen development team for a little while. During this time I added two features.
The first was the ability to send out midi commands. Until that point Hydrogen was a sample based drum machine and had frequent requests to include midi out. The UI is a little rough and the font doesn't match, but nobody seems to be complaing.
I also wrote a virtual pattern implementation. It's fairly common to layer patterns together and add or remove subsets of those patterns. Rather than always explicitly including associated patterns, I added the ability to create a new meta-pattern type that when included will implicitly include those bundled within it. I find it a useful feature.
The Hydrogen website can be found here: http://www.hydrogen-music.org/.
ALSA is a Linux kernel component that provides device drivers for sound cards, audio and midi interfaces, and related devices. In 2008 I provided a modest contribution to ALSA by implementing USB support for the Roland SonicCell.
While it should be included in most flavours of Linux, the ALSA website can be found at http://www.alsa-project.org/ main/index.php/Main_Page.
My original patch submission can be seen at http://mailman.alsa-project.org/ pipermail/alsa-devel/ 2008-May/007908.html.
The change logs for when this patch was included in ALSA can be seen at http://www.alsa-project.org/ main/index.php/ Changes_v1.0.16_v1.0.17 and http://marc.info/?l=linux-kernel&m=121602662531525.