As I mention in the custard-based ice cream recipes, egg whites can be refrigerated or frozen and used later. If you refrigerate them they should be used within a couple days; in the freezer they will last for a couple weeks.
Now there are quite a few ways to use them up. One is to simply beat a couple into your omelet or scrambled eggs in the morning. Another is meringue topping for a pie, or light meringue cookies (like French macarons). Coconut macaroons are always delicious (and easily frozen for a make-ahead dessert). Egg whites are great for sealing homemade pierogies or raviolis. My favorite way to use up a good amount of egg whites is making these utterly awesome friands, or tea cakes, from Ottolenghi.
To say I’m obsessed with Yotam Ottolenghi is a little bit of an exaggeration, but not much. I love that man. The accessibility and ingenuity of his recipes never fails to surprise me. To date, I’ve easily made over 3 dozen of his recipes, and not one has been a flop. This is my favorite of his desserts.