This recipe is the reason I grow Genovese basil in my garden every year!
Slice ripe garden tomatoes and arrange in a circle on a plate, slightly overlapping one another. Slice mozzarella cheese and put a slice on top of each tomato. Put one basil leaf on top of each piece of cheese. Drizzle with a tiny bit of olive oil, and a generous amount of balsamic vinegar. Sprinkle salt and pepper to taste.
Informal version (pictured above):
This can also be made by dicing tomatoes and cheese, and cutting the basil leaves with kitchen shears. Put all ingredients into a bowl and stir well to coat with the vinegar and olive oil.
How much you make will depend on how many ripe tomatoes you have to use, and how many people you need to serve. Be careful--you may want to eat the whole bowl yourself!
Note: This is fantastic with fresh mozzarella. To make it for less, buy a 5 lb block of Mozzarella cheese from Sam's Club (which is under $2.50/pound).
Sam's Club also carries a liter of Balsamic vinegar for under $7.