Built this when I was around 15-16 as a learning project.
Z80 CPU running at 2 Mhz
16 KB ROM
8 KB RAM
Made mostly out of junk I had lying around at the time. The display is from a clock radio, the keyboard is buttons from a scrapped VCR. I didn't have the money or anywhere I could buy components, so I remember hunting old XT motherboards for just the right 74-series logic IC and trying to desolder it without losing too many pins in the process...
I created a small monitor program so it was possible to view memory content, enter small programs and running them etc. I also ended up porting an ancient chess program originally made for the Tandy TRS80, and modifying it so it could scroll the moves on the clock radio display.
It was a huge learning experience building those boards and actually get them to run some software.