Plastic is a scary thing for food storage, because of the possibility of BPA (bisphenol A) and other nasty chemicals leaching from the plastic into your food. And glass or ceramic food storage containers are usually impractical or expensive — or both.
For storing leftovers, my solution is to use canning jars. A serving spoon or pair of tongs fit easily into these wide-mouth vessels. Old canning jars, especially those that are chipped or otherwise unsuitable for canning, can be picked up for something between dirt cheap and free at garage sales or on recycling websites.
The jars can safely go in the refrigerator, freezer, dishwasher, and microwave, and the transparency of the glass helps you keep track of what you have in the fridge. They stack easily, seal tightly, come in a variety of sizes, are fully recyclable, and can hold just about anything.