The best open source alternative to Hydrogen is LMMS, which is both free and Open Source. If that doesn't suit you, our users have ranked more than 25 alternatives to Hydrogen and nine of them is open source so hopefully you can find a suitable replacement. Other interesting open source alternatives to Hydrogen are Qtractor (Free, Open Source), Bosca Ceoil (Free, Open Source), LinuxSampler (Free, Open Source) and drumkv1 (Free, Open Source).
Bosca Ceoil is all about looping, and designed that way at its heart. Songs are built up from lots of tiny 16 note patterns – the intended workflow is that you loop over these single patterns until you have something you like, then you start making variations.
HTML-909 is a very accurate simulation of the classic Roland TR-909 drum machine, created with HTML5 and accessible using any modern browser. Formerly available through its own domain, the app is now available through a GitHub repository, though audio samples are not included.
The orDrumbox is a free drum machine software, creative, with unusual features : auto-composition, polyrythmes, arpeggiator, automatic sounds/track , custom softsynths, lowfi rendering, etc. This tools can compose bass line and complete songs!.