There is a GUI-like application called htop which is pretty decent. (GUI-like here means that it runs in a terminal, but allows you to use the mouse on a PC, so I assume you can use the stylus on Maemo. I don't own a Maemo device yet unfortunately ) It allows you to see what process uses most of the CPU time and memory and kill them. However Flandry is right in saying that you should be careful and not be killing arbitrary processes.