Linux, the popular open source operating system, has been around for over 20 years. Despite being both free and highly customizable, Linux has, for the majority of its history, remained a niche player when compared to Microsoft's (MSFT -1.27%) Windows.

That is, until recently.

Google's (GOOGL -1.23%) Android is on pace to become the dominant operating system worldwide, while the future of Google's other Linux-based operating system, Chrome OS, looks ever bright. At the same time, IBM (IBM 0.06%) is preparing to push Linux more aggressively among business users.

Android could replace Windows by 2017
In April, research firm Gartner predicted that Android would replace Windows by 2017. If current trends continue to play out, that prediction doesn't seem too outlandish.

In just the last few years, Google's mobile operating system has exploded in popularity, and now accounts for about 79% of smartphones and about 62% of tablets shipped worldwide. Given that Google gives Android away for free, and numerous hardware manufacturers have eagerly adopted it, it seems unlikely that Android will cede its dominance anytime soon.

At the same time, sales of traditional PCs -- Windows' bread and butter -- continue to decline. In July, Gartner reported that global PC shipments fell 10.9% in the second quarter -- the fifth consecutive quarterly drop. And don't expect a turnaround in the near future -- Microsoft admits that the traditional PC will continue to struggle.

But even on the traditional PC, Windows could have its hands full. Chrome OS, like Android, is also Linux based, but unlike Android, it was designed with traditional PCs in mind.

Linux desktops, including those running Chrome OS, make up just a tiny sliver of the total desktop operating system market. Still, among laptops cheaper than $300, Chromebooks now account for 25% of the market, and are the fastest-growing part of the PC industry according to NPD (via Bloomberg).

IBM's second $1 billion investment
Twelve years ago, IBM said it would invest $1 billion to promote Linux. That investment led to rise of Linux-based servers, including IBM's own machines. While Linux is not dominant when it comes to servers -- its market share is only about 20% -- it is significant. Back in February, research firm IDC cited Linux servers as a growth area for the industry; with the shift to the cloud helping to drive demand.

Perhaps that growth will accelerate still further. IBM is about to make a second billion dollar investment into Linux, focusing specifically on its Power Systems servers. According to ZDNet, IBM believes that Linux-based Power Systems could bring in new customers, specifically those aiming to take advantage of the cloud and big data.

According to IDC, IBM continues to lead in the server market, taking about 28% of the industry's revenue in the second quarter. Yet, the server market as a whole is shrinking, and IBM's server revenue actually declined about 10% from the prior year.

Part of the reason for the decline may be the growing trend of companies building their own servers. Major Internet players like Google and Facebook have been designing their own servers for years; the social network just built a data center in Sweden that uses 100% Facebook designed-servers.

Perhaps IBM's latest initiative could help turn its server business around.

Microsoft's mistakes
If Linux does eventually replace Windows, Microsoft will deserve much of the blame. In recent years, missteps by Microsoft's management have helped to accelerate Windows' decline.

When the iPhone was announced in 2007, Microsoft's Steve Ballmer actually laughed at it. Ultimately, the iPhone -- along with the iPad -- would serve as the basis of Apple's push into mobile computing, with Google's Android following in Apple's footsteps. Microsoft eventually mustered a response with Windows Phone 7 and then Windows Phone 8, but the delay has led to lackluster developer support; even today, Windows Phone lacks a multitude of popular apps.

At the same time, Microsoft's aggressive redesign of Windows -- in the form of Windows 8's Metro interface -- has led to a backlash among its base of traditional PC users. Some consumers may have found the changes to Windows 8 too drastic, perhaps holding on to older machines instead. Samsung appears to believe that; an executive of the South Korean tech giant told reporters that Windows 8 was about on par with Vista (a previously despised version of Windows that even Ballmer has criticized).

The fall of Windows
In the 90s, the US Justice Department was obsessed with Microsoft's then operating system monopoly. But today, the very notion of a Microsoft monopoly seems silly. In large part, that's due to Linux-based operating systems, particularly Android, as well as Chrome OS and IBM's Linux-powered servers.

As Windows still accounts for about one-fifth of Microsoft's revenue and a sixth of its operating income, further declines in its Windows business could devastate Microsoft's earnings. This is one trend Microsoft investors need to keep a close eye on.