Technically speaking, Linux rarely has “non-responding” processes, as it's almost always possible to send a signal to a process in order to kill it.
You can use the “top” command in a terminal window to list the processes in the system. You can even use this over a serial terminal if a process has taken out the console with it.
As far as system freezes go, if you entire system freezes up then you won't be able to do anything much. It's far more common to have the terminal lock up; you can usually kill an X-windows terminal with Ctrl+Alt+Backspace. If you're using a text terminal then you can also get back to the terminal by sending SIGSTOP (the “stop” signal, used to pause a process); pressing Ctrl+Z normally does this.
If it's just that an X-windows program has stopped working, you can run the “xkill” command, which will change the mouse cursor to a skull-and-crossbones symbol. You can then click on the window that isn't responding and kill the just application that owns that window.
Hope that's of some help,
— Robert J. Lee