Mobile applications markets are expanding at an outstanding rate. Google recently announced that the Android Marketplace has more than 200,000 applications, and, in a similar fashion, Apple has announced that their App Store has more than 500,000 apps.

In the ever-expanding ocean of mobile application development, there are tens-of-thousands of duds: mobile applications with flaws, bugs or little-to-no user base. But, at the top of the heap, there are several small companies that have managed to get their mobile applications (or apps, if you prefer) downloaded onto millions of phones and tablets.

For these lucky companies, many of which are started with tiny teams of 50 or less, comes huge sums of venture capitalist and angel investor money. On several occasions, funding efforts have allowed mobile applications to gain more than $1 billion valuations.

One of those companies is Rovio, the creator of the wildly successful mobile application Angry Birds. By now, you're probably played the game on a friend's mobile device or may have the game downloaded on your phone right now. Well, you're not alone.

Peter Vesterbacka, the CMO of Rovio, recently announced at Slush 2011, an entrepreneurial conference held in Finland, that Angry Birds has topped the 500-million download benchmark. In addition, Bloomberg recently reported that Rovio is seeking funding at a $1.2 billion valuation.

We've been able to get to hundreds of millions of downloads in less than two years. Look at Tetris -- It took them 20 years to get to one hundred [million downloads] or Nintendo -- They have sold 260 million games over the last 26 years, said Vesterbacka during an interview at TechCrunch Disrupt, a tech event in Beijing. It's just that you can go so much faster nowadays.

Mobile games aren't the only things being downloaded at an unfathomable rate. Jack Dorsey, the famous software architect that helped create Twitter, has launched a new mobile payments software called Square. According to a TechCrunch report, Square sought another round of funding earlier this year that gave the company a $1 billion valuation.

While not everyone is reaching the pinnacle of mobile startup success, many players are generating enormous sums of money in parts of the market that are still expanding. For instance, the popular social network game FourSquare is valued at $600 million; JumpTap, a mobile advertising platform, has received $90 million in funds; Ngmoco, the mobile game maker for the iPhone, is rumored to be valued above $100 million. There are many other companies who are raising millions of dollars during every round of funding, many of which you've never heard of.

Though the $1 billion valuation for Square and Rovio is an incredible story, one thing's for sure: In the rapidly expanding, immensly funded mobile applications industry, they surely won't be the last to hit that benchmark.