Mac provides you with Software Update. This notifies you for any update without your being busy looking around sources from web. You can set it to automatically update monthly, weekly, or even daily. But you will not predict what time you will be disturbed by that automatically pop out window, just be prepared. You can update your security, your iTunes and many softwares for Mac.

You can find this software update inside System Preferences or using QuickSilver (Spc) then type soupp, will perfectly perform opening this window. At first time, the cursor will be at the Spotlight panel, a soft touch on key can bring you to Update Software or Installed Update parts (use ← or → to move from one to another).

When Check Now button is pressed..

And this is what happen when you press Cancel after pressing Check Now.