How to crash a system programmatically

KeBugCheck on Windows is the documented way to get a BSOD.

You can also try deleting the root registry key (\REGISTRY) in Windows XP, using the native NT API.

Write and load a kernel module that calls panic() or implement equivalent thereof.

Or simply exec the shutdown or halt command or the syscall that implements it.

If the OS happens to be windows, create a fake driver that dereferences a NULL pointer. Crash!