My ice cube trays recently came out of retirement. They were delegated to the back of the cabinet when my youngest daughter stopped eating homemade baby food, but they've been recycled and turns out they are used more frequently than ever before. What for?
Stock. Making your own stock is one thing that differentiates the professional from the home cook. No commercial stock or broth matches the quality of a chicken, meat, vegetable or fish stock that you can make yourself. It is more flavorful, contains no additives and can be seasoned to your taste. Because stock is the building block for most soups and sauces, it's helpful to have some on hand at all times. Freezing stock in ice cube trays - then popping out the cubes and placing them in a plastic bag - makes it possible to always start with the freshest and best tasting liquids.
Wine. Freeze whatever leftover red or white wine you have in ice cube trays so you'll always have some around for pasta sauce, stews, soups or whatever. I learned that wine could be frozen while I was bemoaning the size of the bottles and, I'm embarrassed to say, the boxes of cheap wine I was buying to cook with. A Safeway employee who worked previously at a Le Cordon Bleu school told me it's standard practice for the chefs.
Pesto. Tomato paste. Meyer lemon juice.
The possibilities are endless. So dust off your trays and start freezing.