For my thesis research, I worked on ChronOS Linux, a real-time addition to the Linux kernel, as well as associated libraries. The ChronOS kernel is built on top of the
PREEMPT_RT patch for the Linux kernel, and allows for writing advanced real-time schedulers which can be be inserted into the running kernel as kernel modules.
Most self-respecting geeks I know tend to leave personal projects in their wake in varying stages of completion. Here is a woefully incomplete and out-of-date listing of mine:
Aedrix, a simple OS kernel which runs on ARM (inclusing the Raspberry Pi) and x86.
One project which I would love to actually work on if I ever had the time, can be found at acal.aclindsay.com.
I volunteer some of my computers' idle time to the World Community Grid. It's easy, and helps to do things such as "find drugs that can disable three particular proteins associated with neuroblastoma, one of the most frequently occurring solid tumors in children" via the Help Fight Childhood Cancer project.
My github page has several small projects I have worked on over time.