Inkscape

If you click ”New”, a box appears before the filter's name and a number after it, below the #. The latter number shows how many objects have that filter applied.
The checkbox is where you can switch on and off a filter for a selected object.

Some important notes:
· a filter definition can be empty, meaning if you apply it to your selected object it will be rendered invisible -as shown in the example.
· you cannot add more than one filter to the same object.
That is a real bummer, you cannot stitch two filters together with the editor at the moment.
Only workaround is to group the filtered object, so then you can add another filter to the group.
(And with groups there comes the possibility of messing up transformations.)