In the Themes->Sidebar Widget you see in the available widgets section the SkypeWidget, which you can drag and drop on your side bar.

You can add an Title which should be displayed in the title of the widget.
Add the user account which should be used to retrieve the online state and functionality.
Select which functionality should be visible on the widget.
This widget creates the button html code which is also created from the skype button page
and uses the buttons created by the skype pages.
To activate the online state you need to configure your skype account to allow this.

Free Download: Download this wp-plugin