Isn't that only a GNU and FreeBSD thing? I think other Unixes will still let you rm -rf /.

On FreeBSD you can do

    sudo dd if=/dev/random of=/dev/mem

and it will do exactly that, write random crap into your memory without any sort of safeguard, causing a spectacular crash and a console that looks like it's having a seizure. Linux won't let you do that unless it's been compiled with a flag to enable full access to /dev/mem and /dev/kmem.