Game studios.
I think that maybe StackExchange qualifies
Apple is killing it on all fronts it competes on, but I wonder, will people continue to pay $800 for a new phone every 2 years, $2500 for a new laptop every 3 years? My intuition says no, but from what I read, Apple is taking more and more market share where it matters, in 1st world countries where there are actually profits to be had. I used to slag on iOs because it was so limited and primitive, but the reality is, they're probably strategically releasing features when they choose to, it's not like they're not smart enough to figure it out. If they're lazy, they could just let Google do all the expensive proving out of features, and then release a better version constantly a year behind and make a killing. Kind of like what Microsoft did for so long, except Apple has much higher volumes.
Mobile and Cloud are where everything is happening. Amazon own(ed?) Cloud, Apple & Google own Mobile. Whole generations are growing up without using Microsoft tech even in developed countries. Which of the major tech companies to arise in the past 10 years use a Microsoft tech stack? (hint: none)
Microsoft tried to take on mobile with Windows Phone 7 and Windows 8 (Surface, Windows ARM, etc). They were all duds more or less and Microsoft was at risk of becoming completely irrelevant.
I think Microsoft's leadership (including Bill & Balmer) took a long, hard look and realized that Mobile has already past the hockey stick growth phase. At that point only the one or two competitors seen as leaders survive; Apple and Google have already won.
They were also at risk of Amazon and other hosting providers eating away at their server business by making it easier to deploy servers at the infrastructure level. Microsoft also critically lost developer mindshare; despite C# being a much better language than Java (and Oracle's mis-steps creating enormous opportunity) no one gave a shit.
There's a thing that happens in large organizations where a leader can become boxed in and unable to change direction. Balmer was clearly in that position and as someone who deeply cared about Microsoft he stepped down and gave Nadela the reigns. You can also bet Bill is backing him or he'd never be able to get away with making such drastic changes.
So what does Microsoft gain? Getting people to use their software. They'll never own the Mobile OS, so may as well accept it and move on.
People don't want to run Windows Servers? fine, run Linux on Azure. They might as well take your money since they sure aren't going to get you to switch. Why did they push Mobile Services so heavily for iOS? Why do they have Android SDKs? Because if they can't own the Mobile OS, they can at least grab a piece by being a really easy to use back-end for the app ecosystems.
And why open-source C# (Roslyn) and the CLR stack? Capitalize on Oracle's lawsuits over Java and Sun/Oracle's mismanagement of Java (and the years of stagnation Java suffered, though that has changed recently). They'd rather sell you developer tools, or make sure that cool new open-source projects start using C# so it makes it more natural to support Windows in addition to *nix/Mac.
Now that it's all MIT licensed and contains full patent promises I consider Mono a completely legit fork. That means even if Microsoft dropped it all tomorrow, Mono could still continue unfazed.