Well, first of all, while it’s true that Delaware serves as a convenient location for many companies due to its friendly corporate regulation and taxation laws, many of those companies keep more than just a post office box for receiving mail. Many banks have quite a substantial workforce there.
There are also some big chemical companies with a significant presence in Delaware, most notably Du Pont. Hell, if the state had been named in the late nineteenth century, it would probably be called Dupont; you can hardly turn around in the Wilmington area without running into something with the Du Pont name on it. I think there are also some big drug companies in Delaware.
Of course, much of this stuff occurs in places like Wilmington. If you move beyond the larger towns and cities, Delaware sometimes becomes almost indistinguishable from similar regions in neighboring states. A significant part of Delaware’s real estate is located on the so-called Delmarva peninsula, a large peninsula that inexplicably has been divided between Delaware, Maryland, and Virginia (hence the name, Delmarva). Whichever state they happen to live in, the people on this peninsula often do similar things—farming, fishing, etc.—and they have more in common with one another due to their geographic proximity than they do to the capital cities of the states they happen to belong to.
Another thing Delaware has is tax-free shopping. The large outlet malls along the interstate attract cars and even tour buses full of people from New Jersey, Maryland, Pennsylvania, and even further afield to shop.
Delaware also makes considerable revenue through highway robbery. Almost literally. The Delaware section of the interstate linking DC and New York/Boston is the most expensive part of the trip. While you pay around 4-5c per mile to drive on the New Jersey turnpike or I-95 in Maryland, it costs about 18c per mile in Delaware (these stats are from an article that is a few years old; not sure if it’s changed since then). Delaware politicians can get away with this because the vast majority of people using the interstate come from out of state, and are just passing through on to way to or from New York or Boston or Washington. Thus, there is no political downside to overcharging, and these out-of-state drivers pay a substantial amount into Delware’s coffers each year.
There was a great article in the New Republic a few years ago called “Delaware: The Worst State.” Unfortunately, i can’t find it online anywhere.
Edited to add:
Here is a Salon article that makes some similar points about the way that Delaware’s tax policy’s have effects well beyond the state’s borders. Also, you’re a New Rebpublic subscriber, you can read the article i referred to above here.