The Seville Orange (Citrus aurantium), also called Sour Orange, Bitter Orange, or Bergamot Orange was at one time widely planted as a decorative tree in the Desert and other warm-climate areas, including Southern California and Florida. They're still to be found in older yards and neighborhoods, and while now often disparaged as being a waste, they're actually a useful tree with a fascinating history.
Citrus aurantium originated, and is still to be found wild, in Southeast Asia, and from there it was carried to the Arabian Peninsula. By the end of the First Millennium, it was in Europe, and for 500 years it was the only orange in the West; its common name of Seville Orange came from its intensive cultivation around Seville, in Spain. Christopher Columbus is believed to have planted the first seeds of the Seville Orange in Hispaniola in 1493, and from that and subsequent plantings the tree spread around the Caribbean and throughout all the suitable New World areas.
It is almost impossible to eat the raw fruit of the Seville Orange, a fact I verified for myself a few years back; that was a stomachache I will never forget. But cooked, or used as an ingredient, the fruit is as useful as any.
Seville Orange is the traditional source of Scottish orange marmalade (recipe here), which is both excellent and ridiculously easy to make at home; I make marmalade each Spring and give much of it away to people who either love it or are too polite to say otherwise. The fruit is widely used in Cuban, Tamil, and Yucatecan cooking (recipe here), and the juice can be used as a sort of "lemonade" when mixed with sugar and topped up with sparkling or still water. The peel is the source of several liqueurs, including Curaçao, Grand Marnier, and Triple Sec, and can be used to make a very good candied orange peel, though you need to blanch it more than with other citrus peel.
The flowers of the Seville Orange are extremely fragrant, the most fragrant of all the citrus blossoms. Those flowers are the origin of the perfume ingredient called "Neroli oil," one of the most widely used scents in perfumery. They can also be picked and then dried in white sugar to make a scented sugar for cakes and pastries, though I've never done that myself; marmalade is one thing, but you have to draw the line somewhere. The flowers are also the traditional orange blossom of the bridal bouquet.
And last, but by no means least, even if you don't care for the Seville Orange, you may already have one and not even know it: the Sour Orange is widely used as a rootstock for grafting other citrus varieties, and is probably the best rootstock for citrus in Arizona. When folks ask me why their previously sweet orange is now bitter and inedible, 9 times out of 10 it's because the top of the plant has been damagd or overpruned and the rootstock has sprouted and replaced the grafted portion.
As a landscape plant, the Seville Orange is, once established, a moderate water user, and provides deep leafy green foliage, brilliantly fragrant Spring flowers, and decorative fruits. It tends to be low maintenance and has a very long life-span. If you're looking to plant a new citrus tree, the Seville Orange probably isn't your best choice, if only because the fresh fruit is inedible. But if you or a neighbor already have one, it's well worth keeping, using, and enjoying.