Whether you're using canned tomatoes for a recipe or cooking with fresh tomatoes, you may find yourself with some leftover juice on hand. Typically, it goes down the drain, but the next time you go to drain a can of tomatoes for a casserole, pour it in a container and save it in your fridge. Because the question isn't so much "What can you use it for?" but rather, "What can't you use it for?" There are countless dishes that you can prepare with this delicious and vitamin-packed juice.
Any kind of rice, couscous or short grain pasta like orzo would benefit from a little tomato juice flavor mixed in with the water. Mexican rice will especially sing from the added flavor. Same goes for many soup broths -- just add the juice and cut down on the water by that same amount. Even the dorm room special ramen noodles will taste better made with water mixed with tomato juice. You can also add the juice to any tomato-based pasta sauce and more exotic dishes like seafood paella. All kinds of sautéed vegetables would taste great with a dash of tomato broth in the pan as well.
If you drink tomato juice, when you rinse out the bottle to recycle it, you can use that water to add small amounts of tomato flavoring to dishes. And there are plenty of cocktails that go with tomato juice, with the Bloody Mary topping that list. Try making your ice cubes from tomato water to impress your brunch guests. And last but not least you can water down the juice and use it to feed your plants. They'll appreciate the nutrients as much as you do.