If you use a right sidebar, you can create a custom widget which will allow you to choose
By making a custom widget and having this HTML in it:
</div> <div id="bottomWidgets" style="clear:both">
What this does, is by adding a closing DIV tag its ended the sidebar DIV early.
Additionally it wraps the remaining widgets in a DIV with ID of 'bottomWidgets' so you can style them completely different from the rest in the CSS.
To choose what widgets go to the bottom, they only have to be placed below this custom widget in your admin panel. Any widget below this custom widget will be sent to the bottom since it is outside of the sidebar.