Hydrogen is an advanced drum machine for GNU/Linux ....
let's break that down :
Hydrogen : is just a really cool name for a program
Advanced drum machine : it's a drum computer (no matter what they say ;-)
for GNU/Linux : well it's not just for Linux, there are Mac/Win ports
Last year i spent some time working on the manual and new site of Hydrogen. I updated the manual and also wrote a case study about the hydrogen setup we use live-on-stage with our band Cocasse.
From time to time i dive into the code (and almost drown) but i mainly focus on the usability of Hydrogen.