Reset object rotation back to 0 degrees

I'm not aware of any elegant solution, but it can be done with the XML Editor.

1. Select the rotated object.

2. Click the XML Editor button on the toolbar.

   

3. Selected element will also be selected in the XML Editor. Find the transform property on the right and click it.

   

4. Change the value to matrix(1,0,0,1,0,0) and click Set.

This will reset all transformations applied to the selected object.