There can be more than one variety of peach, nectarine, plum, or apricot on one tree, each variety ripening at different times throughout the season. For one company in Australia, Fruit Salad Trees have become a business.
Stone fruits, like peaches, apricots, plums, and nectarines, have a large seed in the center that resembles a stone. This family of fruits can easily be grafted together to create a tree that produces more than one fruit.
These can be put in a greenhouse in cold weather areas to be kept year-round, and can also be grown as small bonsais. There are trees with varieties of fruits to suit warm/temperate climates, with other varieties for colder climates.
Grafting different apple trees into one will produce apples year-round. Here is a quick video on the apple Fruit salad tree.
Orange trees with lemons, limes, and grapefruit.