Magic SysRq Key
This should get you started.
The magic SysRq key is a key combination understood by the Linux
kernel, which allows the user to perform various low-level commands
regardless of the system's state. It is often used to recover from
freezes, or to reboot a computer without corrupting the filesystem.
Its effect is similar to the computer's hardware reset button (or
power switch) but with many more options and much more control.
You can plug in a keyboard and type this stuff in blind (with no monitor.)
No filesystem corruption, no extra hardware required.
Edit in 2021; Raspberry Pi 4 with Raspberry Pi OS:
The SysReq key, by default, seems to be Right Alt + PrintScreen held down together.
The graceful shutdown key sequence is "REISUO".
So, all together:
Keep holding down Right Alt + PrintScreen, and with your other hand, press the sequence keys, leaving one second wait time between each keystroke: R + E + I + S + U + O
As Rizzle Light suggested, you could grab some sort of button and wire it to the GPIO pins, and do a little programming magic to make pressing the button execute the "poweroff" command (with admin privileges, of course.)
There are tons of GPIO programming tutorials online that you can adapt, like this one.
Rahul R Dhobi has a nice two-liner python script that will shut down a Raspberry Pi:
os.system("shutdown now -h")
execute your script with root privileges.