The BBC Micro also allows you to change the address of the video framebuffer. A particularly neat trick was to remap it to 0x0000, so you get to see the OS workspace, stacks, program storage etc. You can watch the bits twiddle in realtime as you do things.

There's another neat hack which would cause the machine to run in slow motion. I don't know how it worked; I can't find any references now. Possibly it overloads the system with interrupts. But under its influence, clearing the screen would take several seconds. These two hacks combined beautifully, letting you see all the details of, e.g., Basic's heap management.