I tried out a few at ethereal.org. I'm sure there is utility here, but I'm wondering what the security implications are.

In addition to being able to access system information (username, OS / kernel versions -- which could be used to determine additional suitable attacks, perhaps exposed back to the originating server through query parameters to some RSS/XML request), there is a JavaScript control for doing limited scripting operations.

This feature can be turned off by disabling plug-ins from the Security tab in Safari's preferences. Of course, that will disable Flash as well.