AdWords API specialist Thomas Steiner created an open source ajax gui (there words, not mine:), for all those who want a stand alone app (now up to version 1.0.1), and has recently updated it.

From the Adwords AJAX blog the new version includes
<quote>
a more compact UI design, more consistent icon layout,
a WYSIWYG creative editor,
better support for Javascript multi-window interface,
support for creative exemption requests, and
better support for many browsers including MSIE, Firefox, Safari, Opera, Konqueror, and Camino on the Linux, Windows and Mac platforms.
support for the AdWords API sandbox.
</quote>

Link:
http://adwordsapi.blogspot.com/2006/08/apilitax-101-released-ajax-gui-for.htm:

ps. isn't AJAX GUI kinda of an oxmoron?

You mean oxymoron? Why? I don't see any contradiction.

When I think of AJAX, I think of a dynamic web page; on the flip side, when I think of GUI, I think of a standalone app.

To me "GUI" just implies buttons, comboboxes, input boxes and other widgets like that. Graphical User Interface be it on the web or on the desktop...