By far one of the most satisfactory things (and also hair reducing things) I've had as a long term hobby is: O/S development. Been doing it seriously for just over a year now, it's slow because it is purely free time. Was working happily on the kernel but decided to go back and build a bootloader after I got to higher half kernel and was messing around with the GDT base address overflow hack. It's now turned into a multi-stage boot loader, because I appreciate the idea of your system have full control over your own partition, nothing else.

I enjoy going to http://wiki.osdev.org for a broad overview and then hitting the manuals for the details; sometimes that manual and note book session can take up to 3 days of my free time haha.

I especially love writing custom tools around the development process; I like to call it reducing reliance - I found that this really helps solidify concepts in my mind; although making the process even slower, I feel comfortable at every stage.