A fork-bomb is a good test of a Unix-like system.  You will probably want [kill_all.tcl] if you want to test it out.  If your system fails to work even as root after starting it then you have problems...

One way of doing it in sh is:
#!/bin/sh
$0 &
exec $0

Another way in C is:
#include <stdlib.h>
#include <sys/types.h>
#include <unistd.h>

int main () {
	while (1) {
		fork ();
	}
	return EXIT_FAILURE;
}