Five years ago today, Google and its partners in the cellular service provider and mobile hardware industries unveiled the Open Handset Alliance and the development of Android.

Google's stewardship of the mobile operating system dated back to its 2005 purchase of Android, Inc., a company founded in 2003 by executives including Andy Rubin. (Rubin still oversees development of Android as a Google senior vice president.) But November 5, 2007 was the date Google announced its intention to take the smartphone market by storm.

It wasn't an overnight success. The iPhone had already been available since mid-2007. Android 1.0 software didn't come along until September 2008, and it wasn't until late 2009 that Google and its partners started hitting their stride with blockbuster products like the Motorola Droid.

When I wrote about Android's launch in November 2007, I could not have predicted it would be the world's most widely used smartphone operating system today. But it is—and it's also the second most widely used tablet operating system, with popular pieces of hardware like Amazon's Kindle Fire and Google's own Nexus 7.

There are lots of reasons Android has become a current powerhouse. On its fifth birthday, here are five things we love about Google's mobile operating system.

It's open source

Google makes all the source code in Android public, releasing it under the Apache License, with any modifications made to the underlying Linux kernel released under GPL v2. This might not mean much to the average user, but it's a big deal and the beginning point of Android's unusual level of openness.

Linux and open source software have long played a major role in data centers and business applications. But Linux's inability to take market share from Windows meant most people were unlikely to use an open source operating system—until Android came along. While the majority of successful Android hardware has been explicitly blessed by Google (which sometimes takes a heavy-handed approach with partners), the source code's public release allows anyone to use and modify Android as they see fit.

People who buy the Kindle Fire, or any Barnes & Noble Nook tablet or e-reader may not even realize they're using Android—but they are. For companies not already tied to a mobile platform, Android is practically the default operating system for launching new hardware. A new game console called Ouya uses Android, and even makers of kids' tablets rely on it.

While some complain that Google exercises too much control over Android relative to other open source projects, Android's openness is still the platform's defining attribute. Many of the best things about Android that we'll describe in this list flow from the open nature of the platform.

Widgets make home screens informative and interactive

As an Android user, widgets are probably my favorite feature. Widgets provide a portion of an application's functionality on an Android device's home screen, allowing the user to see real-time updates and manipulate the app in limited ways. On the home screens of my Nexus 7, I can scroll through recent e-mails and read their subject lines, see upcoming appointments on my calendar, or check the weather without actually clicking into the weather application. And with the Google Music widget, I can hit play or pause and move from one song to the next. None of these things are possible with the iPhone or iPad, because Apple's iOS displays only icons to represent each application. They always look the same.

Adding widgets is easy, and you don't have to hunt around for them. From the home screen, you click into the app drawer, and select the list of widgets. Many are pre-installed by Google—more examples include a view of your browser bookmarks and your photo collection—and others are installed along with any apps you download from Google Play. There are also a variety of apps designed to help users create more widgets not enabled by default.

Multiple app stores and sideloading, no rooting required

If you're not a fan of the closed app store model seen in iOS and now the Windows Store, Android is the platform for you. By default, Android does not allow installation of applications from any source other than the Google Play store. But you don't need to root or jailbreak your Android phone to open it up beyond the officially sanctioned app store. That's because Android provides an easily found escape hatch within its security settings, letting users check a box allowing installation of apps from unknown sources.

The most well-known third-party app store for Android is Amazon's, but it's not the only one. In addition to some little-known ones that act like traditional app stores, Mozilla is rolling out a new browser-based app market within Firefox.

Checking the "unknown sources" box also allows side-loading, the ability to install an application from anywhere, not just from an "app store." This comes in handy for me because Swype isn't available on Google Play. It also has been beneficial to customers because of the restrictions carriers have used to hobble Android. Verizon, for example, filtered tethering apps out of the Android Market last year for users accessing the store from Verizon devices. But those same tethering apps could still be installed by downloading them from the Web.

Customizability

In addition to being able to choose from hardware of all different shapes, sizes, and prices, Android users can customize each phone or tablet to their hearts' content. Don't like the way your home screen looks? Download a third-party launcher that completely overhauls the look and feel of your home screen. There are various other ways of customizing user interface components and settings. For example, I was unsatisfied with the battery life of my Nexus 7, which was draining quickly while in sleep mode. But I solved the problem by downloading an app called "Auto Airplane Mode," which puts the device into airplane mode each time the display is turned off, while quickly turning the wireless functions back on the next time I light up the screen. If your battery life can survive it, though, Android's extensive multi-tasking and allowances for background tasks can be very useful.

Beyond those kinds of tweaks, Android lets users change the default programs for e-mail, Web browsing, and other tasks. The openness of Android is especially important in terms of Web browsing. iOS places technical limitations on third-party browsers, so Google's version of Chrome for the iPhone and iPad is basically just a wrapper around iOS's version of WebKit (and it's slower than Apple's Safari). But on Android there are no such restrictions. As a result, users have a real choice of browsers. Firefox has become my favorite and default browser on Android because I like its interface, and—according to the SunSpider benchmark tests I've run—it's just as fast as Chrome.

Various aspects of Android help developers provide unique experiences for users. Android's "Intents" system, for one, makes it easy for applications to interact with each other. Android developer advocate Alexander Lucas writes that, for example, this makes it easy for an app to scan a barcode without requiring the developer to write a ton of extra code. It also helps build a common sharing infrastructure to avoid one-off integrations that can lead to a confusing user interface. Android also implements default protocol handlers in a way that allows deep integration between apps and the Web. For example, this would let an Internet Movie Database application open any link pointing to IMDB.com.

For users who are really serious about customizing their phones and tablets, there is always rooting, which provides almost full control over the Android operating system and more extensive ways of backing up and restoring your device. While rooting is a major part of the Android experience for enthusiasts, one of the best things about Android is you don't have to root in order to customize the user interface and install applications from third-party sources.

The Android Community

In the past five years, a vibrant user and developer community has been built around Android. If you're a "n00b" looking for help fixing, tweaking, or customizing your device, websites like the CyanogenMod forum and XdaDevelopers are filled with plenty of more experienced users willing to help out. Various blogs like Android and Me and Android Police are also great sources for those new to Android.

The community exists at least partly because users and developers are taking advantage of Android's open source nature to change it to suit their own needs. One popular activity for owners of rooted Android devices is to install a custom ROM, essentially replacing the entire operating system with a modified version. Availability of Android source code, of course, is crucial for developers building custom ROMs such as CyanogenMod. This is useful for people who want specific functionality not available in Android by default, and also for those unlucky souls stuck on a phone the carrier refuses to update.

Custom ROMs aren't the only reason for Android's active user community, but they are a good example of why the Android community is important. If you've got a problem with Android, or just something you'd like to work a bit differently, there's bound to be someone to help you out.

Here's to five more years of Android

This wasn't meant to be a comprehensive overview of everything Android, but a sampling of the things we think make it successful. What do you find most intriguing about Android? And what would you like Android to accomplish in the next five years?