I want to kill all processes on my computer. Which command can I use to do so?
killall5 -9 will forcefully terminate all running processes except your login shell, init, and kernel-specific processes.
More information here: http://linux.about.com/library/cmd/blcmdl8_killall5.htm
The easiest way is to use the Magic SysRq key : Alt+SysRq+i. This will kill all processes except for
Alt+SysRq+o will shut down the system (killing init also).
Note that you may need to set the keyboard to XLATE mode first : Alt+SysRq+r
Also note that on some modern keyboards, you have to use PrtSc rather than SysRq.
In some Linux distros, you can switch to Run Level 0 - which I think is halted, but still switched on:
sudo telinit 0
I've actually heard of this being used for dedicated firewall servers since it keeps some of the needed low-level kernel stuff loaded like iptables... weird eh? See here for more info.
To see which distros do what at each runlevel, have a look here.