You may already know this, but you need to specify the widget’s class name as the $widget_name variable in the template tag. For example,
Depending on the widget, you may also need to add parameters to the_widget() to get it to display properly — see notes on thewidget() $instance and $args in the codex. http://codex.wordpress.org/Function_Reference/the_widget
Alternatively, you can use a plugin like Widgets on Pages, which will let you include a widget anywhere using by using a shortcode like this: [widgets_on_pages id=WidgetName]
http://wordpress.org/extend/plugins/widgets-on-pages/