There are many things you can do with leftover whey (even the small amount of whey that accumulates in a container of yogurt) other than to just make whey ricotta. That liquid (whey) is chock full of nutrients like proteins, vitamins and minerals, and has far more sustainable uses than just dumping down the drain.
Whey falls into 2 classes: sweet whey, and acid whey, and both have many uses. Sweet whey is the whey drained off a cheese process that uses a bacterial culture, such as hard cheese (like cheddar, and many soft cheese types). However, if you have added vinegar, citric acid or lemon juice to the milk (as in making mozzarella, or whole milk ricotta) the whey is already very acidic, hence makes acid whey.
Sweet whey contains active bacterial cultures and is great to add to a jar of fresh chopped vegetables to ferment, like sauerkraut or pickles (lacto-fermentation). You can use it as a substitute in any recipe that calls for buttermilk, or to replace the liquid in a bread recipe. In addition to live bacterial cultures, sweet whey is full of vitamins and minerals, and makes a healthy addition to soups, or beverages like smoothies. A common use in Italy is to use the sweet whey to make traditional ricotta, which is different than a whole-milk ricotta. There are other whey-based cheeses, too; consult a cheese book like Home Cheese Making by Rikki Carroll, or The Cheesemaker's Manual by Margaret P. Morris if you can find a copy.
Acid whey is good to feed acid-loving plants in the garden; I use it on my blueberry bushes where there is a constant battle to keep the pH low enough for the blueberries to survive. You can add to the soaking water for beans and grains to reduce phytates (as long as it is rinsed away the next morning). It is not good for making whey ricotta.
Both sweet whey and acid whey may be fed to farm animals, or the household dog and cat. My cats don't much care for the acid whey, but my sister's dog does. Sweet whey can be used for all the same things as acid whey, but not the reverse.