If you want to get into a low-level, open source project that is relevant today, I would try RISC-V (http://riscv.org/). It's an ISA and family of processor cores designed to be competitive with the niche that ARM usually fills these days, but fully open source, with a freely implemtable ISA. Now, it's pretty new and you can't buy RISC-V chips yet, but it's done by a team led by David Patterson, who is one of the fathers of the RISC architecture, and it looks pretty promising as a new open ISA and family of processor cores.
Or if you want to work on hardware that is actually available today, ARM would probably be your best bet. Maybe try porting Linux, or an RTOS like NuttX (http://www.nuttx.org/), to an ARM SOC that it doesn't yet run on.