Banana slicer and hard-boiled-egg slicers have been common in the lower age classrooms at the Montessori schools I have subbed at. The children get to practice peeling the bananas and eggs, then slice them safely with the slicers and have a healthy snack.
I've used our melon baller to ball an actual melon a couple times, but mostly find that too tedious to bother with on a regular basis. My more regular use of the melon baller has been to shape cookies. The chocolate chips cookie recipe I use is a stiff chilled dough, so the baller scoops and releases the dough cleanly, and I end up with uniform, perfectly round cookies to boot.