I bought some HP 4N54 hexadecimal displays some time ago at a surplus store. I plugged them into my breadboard just to check them and they all worked fine so I will use them as my debug displays when I start construction. This device fits the purpose very well for a couple of reasons. First of all, it has built-in BCD decoding logic so no additional hardware is required to display 4-bit hex (0..F) as in the case of regular 7-segment displays. Secondly, it is TTL compatible, no glue logic here neither. Also, it has a built-in latch and is able to store the value until reset. Finally, it is tiny and solid (glass/ceramic package) so I can plug them in and out without fear of destroying them. For the interested, the full datasheet for the 4N5x family may be found here.
In the photo on the left is one of 4N54s displaying 9 (binary 1001 hardwired to its input pins). I am not sure if it’s true but I have read somewhere that these devices were primarily designed for military purposes and some of them were even installed onboard NASA space shuttle.
I will now put them in my drawer in a hope to use them in a couple of months during my first boot-up attempts.