With over 140,000 apps in the Apple App Store, plus 30,000 in the Google Android Marketplace, 2,000 in BlackBerry App World, and 2,000 for Palm's webOS, there is clearly no shortage of smartphone apps.
But how do you know what makes the best apps, the most worthwhile to either develop or use? There are many applications that hardly take advantage of smartphones' capabilities. In the spirit of Paul Graham's essay on How to Disagree, I'd like to propose an App Hierarchy for smartphone apps, from least to most worthwhile. I'll use a college campus map app as an example.
AH0: A piece of paper could do this
Some apps are simply static lists of quotes, principles, or items. This app might be a static picture of a map of a college campus.
In this case, you'd do just as well carrying around a note card with the information. Granted, having the information on your mobile phone saves space and paper, but an app seems like overkill here.
AH1: A web site could do this (nearly) as well
Some apps provide functionality that would be delivered fine in a web page. For example, this app might let you click to pull up a few different maps for different areas of campus.
In this case, a couple web pages, or a couple clicks in a simple interface, would work well. You could release an iPhone web app (and get it listed on Apple's web site). Perhaps the advantages of such an app, as opposed to a web site, are speed and not having to depend on a connection (either Wi-Fi or cellular).
AH2: A non-mobile computer could do this
Some apps use computing power but would be just as functional on a non-mobile computer. For example, this app might let you manually input a position on the campus map, then pan and zoom to see what the campus looks like.
While such apps take advantage of the computing power of a smartphone, they could run just as well on a desktop or laptop computer. While portable computing power lets people run apps on the go, if computing power is all you need, your app would be better suited to run on a full-fledged computer with more resources. Only when portability is key to the app's use case does it really make sense to code it as a smartphone app, rather than a web page or desktop application.
AH3: The portable nature of the phone makes this easier or more convenient
Some apps could theoretically run just as well on a non-mobile computer but are significantly more convenient on a portable device. One example is a campus map app that gives you turn-by-turn directions from a position you manually input to a destination. Another example is an expense-tracking app that lets you input purchases as you make them so you don't forget to do so when you get to a non-mobile computer. Many games also fall into this category because they let you do something fun while you've got a few minutes to kill.
The main advantage here is the portable nature of the smartphone: It lets you access information when you're on the go. It also lets you pull in information just in time; for example, the map app might pull up a picture of your destination building as you're approaching it. These apps let you do something that you'd like to do in a place where you wouldn't have access to a computer.
AH4: The portable nature, connectivity, and computing power of the smartphone makes this possible
Some apps take advantage of the computing power, portability, and networking capability of a smartphone -- for example, a campus map app that lets you manually input your location, shares it with selected friends, and gives you directions to meet them.
Here, you're taking advantage of multiple facets of the smartphone. Its wireless networking lets you share information. The phone's portable nature lets you receive timely information, such as directions, while you do the task (walk to meet your friends). And its computing power makes possible the advantageous integration of these parts with a speedy interface.
AH5: Only a smartphone could do this
The most advanced kind of smartphone apps use all the key components of the device: computing power, networking, and smartphone-specific extras such as an accelerometer, compass, GPS, or camera. (Granted, many computers now have cameras, but they're not easy to point at any given object.) One example is a map app that finds you by GPS, reorients itself based on which way you're facing, tells you what classes and events are going on in nearby buildings, and tells you if the dining hall is open.
Here, the smartphone is not just a desktop computer wannabe; its unique characteristics let you do something you couldn't do with a non-mobile computer. Its GPS chip and networking capabilities combine to provide added value, namely the ability to determine where you are and where to go. Its compass lets you reorient the map to reflect the direction you're facing. It knows where you are and what time it is, so it can provide information you can act on here and now.
Why the smartphone app hierarchy is important
The development of new technology, devices, and media with unique uses moves us forward. For example, when graphical user interfaces came to computers, WYSIWYG page layout applications became feasible; the previous method of page layout (typing in the text on a computer, then laying it out on a mechanical device) became obsolete.
In this sense, the smartphone is a medium which should be treated as distinct from paper, a feature phone, a web page, or a non-mobile computer. The question is, which is the best medium to accomplish what you want to do? If the answer is a piece of paper, a web page, or a non-mobile computer, a smartphone app may not be the best choice. Similarly, as tablet computers become more popular following the launch of the iPad and Android devices, developers and users will have to determine what they are the best medium for.
I believe smartphones will have the greatest impact when their apps let us do things we couldn't do before. While I wouldn't begrudge any gamer the enjoyment of playing Sudoku on her smartphone, or the developer the income the app brings in, moving up the app hierarchy will let us accomplish things that are only possible in the smartphone medium.
Image credit: iPhones: Idea go / FreeDigitalPhotos.net.