I'm not big on organizational gadgets (as anyone who has seen my closet can attest) but this product has actually transformed our fridge: LabelOnce Erasable Food Labels. You put a label on your container once, and then can erase and reuse it for whatever the contents currently are. Great for labeling bottles and food containers that are going to daycare, and also for keeping track of all those little containers of kid-sized snacks.
One small packet includes 70 labels of various sizes, and comes with a fine-point Sharpie and a permanent ink eraser. The labels can go safely into the microwave, dishwasher, fridge, freezer, and stay labeled until you erase them. And then they actually erase cleanly and can be used again.
I'm a total fan. No longer are our leftovers scary, uncharted territory.