Hi Colin,
My collection of 8 bit CPU's, 6502, Z80's and 8085's would end up on a bread board hardwired with the NOP instruction and a 555 timer clock to watch a few led's connected to the address and control lines blink away. I once included a few dip switches on the breadboard to enter a basic program. Of course every time I changed CPU I would have to rewire the breadboard, most frustrating.
What I needed was a contraption that could store a simple program in ram using machine code via toggle switches then release the switches so the CPU can control the address and data bus of the ram. My box features a 2K ram chip, of which only 8 bits are used for a maximum of 256 bytes for program storage, more than enough to get a few leds blinking. I used the tristate feature of the the 74LS244 to connect the toggle switches to the ram. In program mode I enter the address and data, store and move on to the next address. Once the program is entered the 74LS244's are switched to hi Z mode and my breadboard Z80 takes control of the busses. The hex display was added to make it a little easier operate, I mapped the 8 bits to 7 seg displays using an eprom. The DB25 connector on my "Micro assist" is used to interface to the CPU, I also built a simple 4040 counter board to automatically address the ram from 0 to 255 and reset.
My CPU's still end up on a breadboard but at least a little easier to get an led to flash
Hope this is of some use.
Regards