Feta cheese can be made from the milk of goats, sheep or cows. This cheese is very crumbly and has a tangy, salty flavor. Part of the process of making feta involves curing the cheese in brine for up to several months; this contributes to feta's distinctive flavor. Feta works well in a wide variety of dishes ranging from fish courses to salads. Depending on the use of the feta, you have several options for substitutes.
Cottage cheese has a different flavor than feta, but its texture is somewhat similar. Cottage cheese works especially well as a replacement for feta when you are looking for pure bursts of flavor, such as when you would sprinkle crumbled feta over a salad. Place the cottage cheese in a strainer and allow the whey to drain off. This is not necessary, but will give the cheese a similar texture to feta. You may even rinse the cottage cheese to remove more of the whey. Add some extra salt to the cottage cheese to get closer to the flavor of feta.
Cotija, a Mexican cheese, may be made with either cow's milk or goat's milk. Its flavor is somewhat similar to Parmesan and it may be used in a similar way. It also works as a substitute for feta because it is much more crumbly and somewhat sharper in flavor than Parmesan. Simply add an equivalent amount of Cotija to your recipe at the same time and in the same way as you would have added the feta.
Ricotta salata tastes almost like a mild feta; it is less sharp and salty, but still somewhat resembles feta in its flavor. Like feta, ricotta salata is easily sliced and crumbled. Feta and ricotta salata do differ somewhat in texture, however. Ricotta salata tends to be silkier and somewhat firmer than the crumbly and hole-filled feta. Ricotta salata can be substituted for feta if you want to keep the flavor of the dish relatively intact while reducing the tangy, salty sharpness of the cheese.