Apple's iPhone application store is as crowded as a Beyonce concert, with more than 20,000 apps available. But one independent developer still managed to rake in $600,000 in a single month with a single iPhone game.

Ethan Nicholas, developer of a tank artillery game called iShoot, told Wired.com he quit his job the day his app rose to No. 1 in the App Store, earning him $37,000 in a single day.

"I'm not going to be a millionaire in the next month, but I'd be shocked if it didn't happen at the end of the year," he said in a phone interview. "If it weren't for taxes I would be a millionaire right now."

Until recently, there has been no realistic way for individual programmers to make serious money on their own. Most of the software market is dominated by big companies, and the traditional distribution method for independent developers — shareware — isn't conducive to striking it rich. By contrast, Apple's iTunes App Store provides a platform for marketing, selling and distributing software; all a developer needs to provide is a good idea and some working code.

Nicholas' success story proves that there's still plenty of potential to strike it rich in Apple's seven-month-old App Store. In September, iPhone developer Steve Demeter said he made $250,000 in just two months with his puzzle game Trism. But as the App Store expanded rapidly, many developers thought the store would get too crowded with apps and business would inevitably slow down.

It wasn't easy for Nicholas, either. After getting off his shift as an engineer at Sun Microsystems, he worked on iShoot eight hours a day, cradling his 1-year-old son in one hand and coding with the other. He didn't have the money to buy books to learn how to write an iPhone app, so he taught himself by reading websites.

When iShoot launched in October, business was slow for a while. And then Nicholas found some spare time to code a free version of the app — *iShoot Lite, *which he released January. Here's how that helped: Inside *iShoot Lite *he advertised the $3, full version of iShoot. Users downloaded the free version 2.4 million times. And that led 320,000 satisfied iShoot Lite players to pay for iShoot.

The game soared to the No. 1 spot — and it stayed there for 26 days. It's only February, so Nicholas is still awaiting payment from Apple, and he couldn't provide documentation to substantiate his earnings. However, Media Bistro reporter Bryan Barletta confirmed that *iShoot Lite * was No. 1 for about three weeks. As of this writing, iShoot sits at No. 6 in the App Store's top 25 paid apps.

Rana Sobhany, vice president of iPhone app analytics company Medialets, said the math made sense and Nicholas' success is very believable.

Nicholas' story shows how a clever marketing strategy can pay off with money and recognition on the iPhone store — and he didn't even have to hire a PR agent. He said the ingredients to his success were simply word of mouth, luck and a quality game.

What compelled him to code an iPhone game? Hard times for him and his family — and he was inspired by Trism, he said.

"I never expected to get anywhere near where I did," Nicholas said. "And of course I've more than doubled [what Trism accomplished] in one month."

Despite nearly becoming a millionaire, Nicholas said he and his family haven't made any lifestyle changes — yet. The first item on his agenda is to hire a nanny.

UPDATE 2/13/2009: Nicholas has posted a response, below, to the many comments on this story.

iShoot Download Link [iTunes]

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Photo courtesy of Ethan Nicholas