The first is to develop widgets as HTML files and test them in Safari. (Apple makes a similar suggestion in their "Developing Dashboard Widgets" article.) While certain features (specifically, the widget object) are not available, it is still the perfect environment to test the UI and any web-based functionality.

Another option for widget developers is to use Amnesty Widget Browser to test widgets outside of Dashboard. Unlike Safari, Amnesty does have a widget object implementation and it's also easy to quit and reload your widget-under-development.