Whether it's a liquid in the fridge or paint in the garage, it can be seriously annoying and time-wasting to open a non-see-through container and find only a few drops. The Marc and Angel blog recommends using plain old rubber bands to curb the frustration:
Mark the Level of Liquid Remaining in a Solid Colour Container - Take a paint can for instance... before you pound the top back on, wrap a rubber band around the outside of the can at the same level as the paint remaining in the can. Next time you need it you'll know exactly how much you have left in a single glance.
Hit the link for 39 more resourceful rubber band uses, and feel free to share your own creative re-purposing in the comments. Photo by Ben Stephenson.