What Mac do you have the problem with, what Mac OS X and Safari and iTunes and Skype version does it use?
When do they quit, during startup, later, earlier?
What steps have you taken before the problem occurred and since?
Have you tried repairing permissions via Disk Utility yet?
Maybe you should also change your thread title to something more descriptive like: Safari, Skype and iTunes keep on quitting on my "insert name and version of your Mac here" using Mac OS X 10.x.y.
To edit your thread title, just click on the
button on the bottom right of your original post and then click the "Go Advanced" button below your message.