I have some pdfs with 6 slides per page, and it's difficult to crop them with exact the same dimensions. A simple ruler (like the one in Word for example) will by a huge help.
And If possible some kind of automatic align. Could be instantaneous: when I draw a second rectangle with borders almost aligned with the first rectangle, the program automatically align them. Or can me done a posteriori: after draw the rectangles and selected the desired ones, an option in the mouse menu "align".