iphone_apps_stackedLego bricks might just be the epitome of modularity. Though over 19 billion of the plastic building blocks are produced each year, anyone could take a brick from this year and, barring any defects caused by parents’ unprotected feet, combine it with bricks from one, two, three decades ago. Owners can mix and match pieces from a variety of sets and themes to create their own Lego-whatevers, allowing them to build outside of the box.

Apps are the Lego bricks for our smartphones, tablets, and PCs. They allow users to create their own ecosystems and expand upon manufacturers’ visions for computing. Do you want a place to hold your thoughts, grocery lists, and to-do items? Snap — or install, whatever — Evernote onto your iPad. Want a cloud-based music service? Add Rdio to your Lumia 920. Fancy the Amazon Appstore? Stick it on your Nexus 7.

Many of these applications and services are available across platforms, jumping from Windows and iOS to Android and OS X as easily as a Lego brick can jump from a “Star Wars” box-set to a “Pirates of the Caribbean” pack. These apps often serve as the sole differentiator between two devices while simultaneously allowing users to take advantage of different platforms without fear of leaving their data behind, as they had to just a few years ago.

A prime example: I received a Walkman for my 16th birthday. Not the “Holy hell, does that thing play cassette tapes?” kind of Walkman, but the “Sony’s trying to compete with the iPod” digital Walkman. I also received an iTunes gift card, as the gifter was prescient enough to realize that I would want to put music on my new mp3 player.

Anyone who owned a non-iPod digital music player before Apple started offering DRM-free downloads in iTunes know what happens next. My gift ended up becoming an exercise in frustration involving a Windows computer that could barely use iTunes, frantic Web searches, a burnt CD, and then, finally, a song-filled Walkman. The gifter may as well have bought me an English-to-Spanish dictionary and a copy of “Les Miserables.”*

It’s much easier to listen to music now. Someone could sign up for Rdio on their Windows PC, install it on their Android tablet and their iPhone, and use someone else’s Mac and have their music readily available on each device. Like a Lego brick, Rdio simply needs to be snapped onto whatever else someone is using. Replace “listen to music” with any computing-based action and “Rdio” with your app of choice and those statements would apply just as well.

Some companies have based their entire mobile strategy around this modularity, whether that’s Amazon’s decision to make its Appstore available to Android devices and fork Android for its Kindle Fire tablets or Facebook’s continued attempts to take over both Android smartphones and the iPhone, to varying degrees of success. While Google and Apple are providing the basic “kit” consumers purchase, Amazon, Facebook, and many other companies are relying on individual blocks that can be snapped on and off of a device with ease.

Mobile operating systems are often referred to as “walled gardens,” but that metaphor doesn’t account for the rise of apps that supplement and, in Amazon and Facebook’s case, replace a platform’s features with another company’s — what Wired calls “apperating systems” — or for users’ increasing ability to move between boxes as more and more apps go cross-platform.

Modern platforms, applications, and services feel more like Lego sets and bricks. While Apple and Google might prefer that users stick to the box they’re given, Amazon and Facebook have shown how users can essentially build their own ecosystems and operating systems with a little bit of effort. Hell, Google itself has started to subvert Apple on its own platform, with its applications handily beating Apple’s own and offering iOS users a mini-ecosystem.

Eventually, as Web-based applications become increasingly capable and cross-platform development tools become easier to use, this application-as-brick metaphor could become even more recognizable. Apps and services could be snapped on as easily as a Lego brick and, luckily for parents but unfortunately for businesses, just as easily removed and forgotten.

[Illustration by Hallie Bateman]