Launched in 2008 across the pond, the simple, legal "all-you-can-eat" music service made waves in parts of Europe with an innovative "freemium" model that lets users listen to over 15 million tracks for free. "The essential feeling we wanted to create was to have all of the world's music available at your fingertips," Spotify CEO Daniel Ek told Fortune recently.

    To that end, Spotify is wicked fast and snappier than Apple's iTunes, despite the fact that all its music resides in the cloud. Users don't have to pay a cent upfront to tune in, but the "free" tier is ad supported and after six months, users will be asked to pony up for one of Spotify's premium plans that scale up to $10 a month. Those plans allows users to store songs on mobile devices.

    Ek believes that presenting as low a barrier of entry as possible and coupling it with a sharp user experience is key to reeling in users. To date, it has more than 10 million users, a reported 2 million of whom pay. The company expects its U.S. launch in July to pay off in a big way by netting 50 million Americans by the end of its first year of stateside operations. Ambitious? Sure. But with Facebook's recently-launched Spotify integration and Silicon Valley's blessing, anything's possible.

Cloud: Dropbox

    Co-founded by Drew Houston in 2007, Dropbox lets users store digital files in an electronic locker the owner can access or share on nearly any Internet-connected device. Its strength lies in its simplicity. Users can interact with their uploaded files via the Dropbox app or website as if they were in any regular computer folder, view photos in Gallery format and share them with non-users.

    Since last January, the service has exploded from 4 million to 25 million users. Now, some 200 million files are saved on the service daily -- despite the fact that Dropbox has spent zero on advertising. As Houston told Fortune earlier this year, it's all thanks to word of mouth. Some 30% of new sign-ups now come via its user referral program, where users get an additional 250 MB free for each successful referral. Another 20% are due to its easy file-sharing features -- that proselytize the service when files are passed around. And despite Dropbox's focus on consumers, it counts hundreds of thousands of businesses among its customers now, including United Airlines, Red Bull and Tishman Construction, the construction manager for One World Trade Center in New York City.

    Up next? Another rumored round of funding between $200 million and $300 million at a $5 billion-plus valuation.

Gaming: TIE

Gameloft

    As Gameloft proves, the recipe to success on mobile platforms isn't originality so much as rapid development and competent execution. Since spinning off from video game maker Ubisoft in 1999, the company and its nearly 4,800 employees has become a veritable hit factory with games some critics might call copycats: N.O.V.A. could be mistaken for Microsoft's Halo and Eternal Legacy bears a striking resemblance to the Final Fantasy franchise made by Squaresoft. But all this probably means little when compared with the bottom line. Last year, Gameloft reported 150 million game downloads with revenues of $198 million.

Rovio

    Who knew flinging cartoon birds across a 4-inch screen could be so lucrative? Since launching as an iPhone app in December 2009, Angry Birds has exploded into a gaming empire. There have been 350 million Angry Birds downloads since the game's launch in December 2009, 150 million more downloads than when Fortune checked in on Rovio last June and more than triple the number of downloads reported last March. Merchandise is moving along at a remarkable clip too. A themed cookbook sold out in the company's native country of Finland -- it's coming stateside in the next few weeks -- and Rovio is selling one million stuffed toys and another million t-shirts a month.

    Indeed, things couldn't be better for Rovio right now, which has rolled out Angry Birds out to numerous platforms including iOS, Android, Nook and Google's Chrome Web Store. It also has at least two new games in the works for release by year's end. Also on tap: an Angry Birds movie that will likely be developed in-house thanks to the acquisition of Helsinki-based animation studio Kombo.

Photos: Instagram   

    Co-founded by Kevin Systrom and Mike Krieger, Instagram is a free iOS app that lets users take photos, give them a retro look with a number of image filters and then share them through Facebook, Twitter, Foursquare or Tumblr.

    Since its launch last October, Instagram has become a popular tool for iPhone users. The six-man team reports 10 million users have added 150 million plus photos. To celebrate, Instagram launched version 2.0 with a neater interface, more filters and performance of up to 200 times faster.

Flipboard：想读什么点什么   

    过去一年间，市场上一直充斥着各种精选网络内容的工具，从美国在线公司（AOL）的数字版杂志Editions和个性化电子杂志Zite，直到坊间盛传谷歌即将推出的新闻阅读类产品Propeller，它们都号称要成为网民的头好选择。这一切并非巧合。

    这类产品大部分都和Flipboard有着千丝万缕的联系。它是由麦克•麦库开发的免费iPad阅读软件。今年早些时候，麦库对《财富》杂志称：“最大的挑战之一是简化内容的发现、消费和理解。”Flipboard是如何解决这一问题的？它将网站、博客、Facebook和Twitter中读者所喜爱的内容用电子杂志的形式呈现出来，用户只需用手指滑动或“轻敲”，就能轻松阅读。

    尽管刚刚起步，但Flipboard正在快速成长。自从其去年首次亮相于“《财富》杂志科技头脑风暴大会”（Fortune Brainstorm Tech）以来，它已被下载了超过350万次，并且几乎每10台iPad中就有1台安装了这一软件。这么多用户每个月“轻敲”阅读的次数约为5.5亿次。实际上，Flipboard已经成为业界的热门话题，它已成为一项标准，所有其他阅读精选类软件都得常常与之一比高下，或许这就是对它最好的赞美。

    It's no coincidence that over the last year, the market has been flooded by a number of tools gunning to become your go-to for curated web content, from AOL Editions and Zite to Google's rumored upcoming offering, Propeller.

    A lot of that has to do with Flipboard, the gorgeous free iPad reading app founded by Mike McCue. "One of the biggest challenges is how do you make that content more easily discoverable, easily consumable, easily digestible," McCue told Fortune earlier this year. Flipboard's solution? Present your favorite content from web sites, blogs, Facebook and Twitter, and in a digital magazine-like format you swipe or "flip" through.

    While it's still early, Flipboard is growing fast. Since its debut at Fortune Brainstorm Tech last year, it's been downloaded more than 3.5 million times and can be found on nearly one in every 10 iPads. Those users make some 550 million flips happen every month. In fact, Flipboard has become so buzz-worthy, it's become the standard against which all other curation apps are often compared to, which is perhaps the best compliment of all.

Utilities: Google     

    When it comes to Android, it should come as no surprise that Google rules the roost. Of the top five Android apps, only one was developed outside of Google, and that's Facebook's. The other four in descending order according to Nielsen: Android Market with 90.5% reach, Google Maps (74.6%), Gmail (74.5%) and Google Search (71.9%).