What are some fun things you can do using the ICSP header? I bought 10 Arduino Nanos for a project, each with all of their headers soldered in place.  Each was going to go into a tiny space, so I found myself needing do desolder the ICSP headers.

This led to a bit of a moral conundrum.  What would I lose by removing these useful looking headers.  They cover quite a lot of space on the nano, so they must be important right?  Am I going to regret removing these things when I finally realize how useful they are (I thought to myself.)

In the end, I did remove the ICSP headers, but the event got my thoughts going.  What is this header all about?  Why is it there?  What can I do with it?  I began to search for things that I could do with the ICSP header, but engaging results were not forthcoming ( except for this project:  https://www.arduino.cc/en/Hacking/ParallelProgrammer )

Anyway, I was more interesting in things that flash, move, jump, sing, and such, so are there any interesting things that can be done with the ICSP header?
You can use every single last byte of flash on the device.
Since you're performing low-level programming of the device you get the entire chip to play on, without having to worry about bootloaders and such.

You can debug (breakpoint, single-step, watch, disassemble, etc.) the code on the chip using software on your computer.
Admittedly the devices you'd need in order to do so are more expensive than a simple ISP programmer and it only works with specific pieces of software, but it can be done.