I often get asked a simple request during a project, and the solution to the problem is sometimes more complex than I originally thought. One of these problems was adding a WYSIWYG editor to the summary field on the node edit form. There isn't an easy way to do this, but it is possible to get a good solution working.
Using the hook_form_alter() hook we can intercept and change the node edit form to change the type of the summary element from a textarea to a text_format element. In order to get the WYSIWYG component of the form working we will need to also add a format to the form element. For the purposes of this example I have created a module called wysiwyg_summary, so the hook is called wysiwyg_summary_form_alter().