Use Vis-à-Vis Wet Erase Markers to label leftovers. They write beautifully on glass (you'll recall that glass is one of the best fridge storage materials), and the ink comes right off with just a little water (and no scrubbing!).
Some other uses for Vis-à-Vis markers:
-leave messages directly on windows/mirrors/fridges/doors for your spouse or housemates
-leave reminders for yourself on things you need to clean (toilets, windows, etc)
-hang an empty picture frame and use it as a to-do list or menu or weekly schedule
-help a preschooler tell time by circling the number on the clock that the big hand will be pointing to when it's time to go
-draw mustaches on framed family portraits
Disclaimer: I have no affiliation with the Vis-à-Vis marker company, and I didn't receive anything in exchange for writing this post. I just like this product.