This question is prompted by some good ideas I've seen here, and a few I've had on my own. I thought it would be useful to collect them in one place.
So the question is, what types of add-ons are useful to include in a new PCB design, which can be either populated or left unpopulated?
I'll prime the pump.
1) A few GND pads. I make them the size to take a 0.025" square pin. When I was stupid I'd lay out boards with no place to attach a scope ground probe!
2) A few LEDs for debugging. One nice arrangement is a single LED hex digit, driven by 4 output pins, but even individual LEDs are very useful. And a place to clip on a scope probe, either on the LED lead, or a separate pin as in #1.
3) Pads on unused port pins. 0.025" compatible is good.
4) Power pads in parallel with the power connector. Sometimes I don't want to add a connector and mating connector, I just want to solder a couple of wires to the board.
I'll stop here - keep 'em coming!