Use Caps lock key to click without 3rd-party software
I don't know any way to do it without third party software, but it is possible with KeyRemap4MacBook and PCKeyboardHack.

First use PCKeyboardHack to change caps lock to F19. Then save this file as ~/Library/Application Support/KeyRemap4MacBook/private.xml:

&lt;?xml version="1.0"?&gt;
&lt;root&gt;
  &lt;item&gt;
    &lt;name&gt;f19toclick&lt;/name&gt;
    &lt;identifier&gt;f19toclick&lt;/identifier&gt;
    &lt;autogen&gt;__KeyToKey__ KeyCode::F19, PointingButton::LEFT&lt;/autogen&gt;
  &lt;/item&gt;
&lt;/root&gt;


Then open the KeyRemap4MacBook application, press the ReloadXML button, and enable the setting.