Build an 8-bit computer from scratch
I built a Z80 computer for my high school senior project in the early 80s.  I got CP/M mostly running; the floppy controller had some bugs I never quite worked out.  Pretty much started me on the career path I've taken.I'm building a 68000 system now for fun.  I know I'll never really understand my desktop, so I'd like to have at least one computer in the house that I really fundamentally understand.