Off topic, but.....
I like westaust55's 15-way D idea as they're not widely used except for PC game connectors and less so these days, I had also thought about those.
for me at least the 15 pin D-connector (the 2 row version not the 3 row VGA type) works well and is readily available in electronics shops.
Within the main "experimenters box, I have items such LDC module, keypad modules, piezo speaker, EEPROMs, SPE030, DS1338 RTC, LED bargraph, etc
plus a number of other digital and analog devices (such as pot, DS18B20, LDR, hall effect sensor, IR LED, etc).
Through the 15 way conenctor using an X1 PICAXE chip, I take +5V (2pins), 0V (2-pins), i2c comms, 1 ADC input, 3 digital input and 5 digital output.
AN X2 part would be even more versatile.
To date the above format has proved to be adequate as a basis for my "expansion" modules.
For other work and for prelim testing it is typically back to a breadboard and often an 18X or 08M.