Software of the decade.

[Smasha]

Operating System: Windows XP. Extraordinarily long lived and still going strong despite the rise of Ubuntu and two subsequent Windows. Came out right at the start of the decade. I believe it is still the world's top operating system, by number of installs on online systems.

Application: A tie between iTunes and Phoenix / Firebird / Firefox. iTunes came out at the start of the decade and was key in changing the way people learn about, get, and play music. I'm not a user, but even to me the way I get my music seems a little quaint compared to iTunes and the iTunes store. Firefox (under whatever name it has had) ushered in a surge of change and innovation in browsers - the browser that came with the OS of the decade didn't even have tabs, and wasn't too different from its incarnations from the 90's. Microsoft just wasn't innovating until Mozilla gave them a kick in the pants, and Firefox set the stage for Chromium. I even speculate that the rise of mini-applications for mobile devices owes a lot to Firefox's popularization of Add-Ons.

Protocol: BitTorrent. I don't think any P2P system had the longevity of BitTorrent. There was once a time when there was no go-to for P2P - BitTorrent competed with eDonkey and Soulseek especially - but now BitTorrent has rather soundly become the go-to.

The protocols for streaming content deserve an honourable mention. But they don't have the name-recognition that the P2P protocols have. The users of the streaming protocols aren't even aware they are using those protocols.

Programming language: Python puts up a very good fight - it has enabled people who don't consider themselves programmers or who are uninterested in the formal sciences (computer science, math) to write novel, useful code. I expect it will become a standard starting place for programming pedagogy. And experienced programmers endorse it too. But the winner here is PHP. It once competed head-to-head with things like Perl, but now it has become the go-to. All that Web 2.0 stuff you hear is really just about the increased use of PHP. Nowadays, websites that don't use dynamic content stand out as being odd and cheap.