Anchors: links are one of the most common objects to add Javascript to. You might want to have something happen onMouseOver and another thing happen onMouseDown or onDblClick. Simply select the link in the document window and apply the behavior.
ex.
<a href="Javascript:;">linktext</a>
<a href="#">linktext</a>
Body: If you want the behavior to apply to the whole page, such as when it loads, you would add it to the body tag by selecting the whole body tag and applying the behavior
Images: You can do something when an image loads or when you onMouseOver it, etc...Just select the image in the document window and apply the behavior.
Forms: You can have things happen when a user presses a submit button, for example, in a form. Like having a new page come up.
Form Fields: You can also have things happen with text fields for forms, such as warning a user that they forgot to enter their email address in a form or that they were supposed to enter numbers and not letters, etc...