David Karp, the 21-year-old brains behind Tumblr, a site that allows users to create Web logs in just 10 seconds, is a fresh face of the rising wave of New York dotcom entrepreneurs.

During the technology bubble of the late 1990s, almost all the technology companies were based in California’s Silicon Valley, but today many Web entrepreneurs are based in New York City.

“There is a lot happening here in New York,” the director of the Columbia Institute for Tele-Information, Eli Noam, a professor of economics and finance at Columbia Business School, said. “A lot of the advertising is here, money is here, and creative people gravitate to this city. Many of these startups are not so much substitutes to old media, but complements to them.”

More than 500 people turn up every month at meetup.com’s New York Tech MeetUp, and Google recently opened a large Manhattan office for its 700 New York-based employees.

Last month, a group invested $750,000 in Tumblr, betting that the nascent Web site, a hybrid between a social networking site and a traditional blob, could become the next Internet phenomenon. “There’s this great wave of innovation happening right now,” Mr. Karp, a native New Yorker and self-described geek, said. “A lot of burn-off from Wall Street is fueling technology companies here.” Mr. Karp began his career as a technology guru at age 14, when he spent a summer working for Fred Seibert, a former president of Hanna-Barbera who is the creator of “The Powerpuff Girls,” at his Manhattan cartoon shop Frederator Studios.

Before long, Mr. Karp was gaining a reputation in the New York tech world as a savvy Web programmer with a sixth sense for what works online.

He started to receive freelance assignments, including for a Web start-up aimed at new and expecting mothers, UrbanBaby.com.

Mr. Karp was a sophomore at Bronx Science when he quit school to work full-time on UrbanBaby, single-handedly running the technical side of the business for three years from his mother’s Upper West Side apartment.

UrbanBaby was a hit. Mr. Karp’s labor earned him some equity in the company, and payday came in May 2006 when publicly traded CNET Networks acquired the site for an undisclosed amount.

“For the first time, I had some money,” Mr. Karp said. He then started his own company, Davidville Inc., and brought on another young programmer, Marco Arment. The two rented space in Mr. Seibert’s Midtown cartoon studio, where they developed senduit.com, a file sharing site, and WorldwideFido.com, “a YouTube for dogs.” Both have been successful. Mr. Karp admits he missed out on a “normal childhood,” but said he doesn’t regret his decision to become a full-time Internet entrepreneur at such a young age. He said he even toys with the idea of applying to New York University. While Mr. Karp may one day go down a different path, for now his energy is devoted to Tumblr. The site launched in March and boasts 110,000 registered users, most outside of America.

“In Japan, we have tradition to value cute things high,” a Japanese user, Yukihiro Matsumoto, wrote. Tumblr is undeniably cute, so Mr. Karp posted the comment on his own blog at tumblr.com.

Mr. Karp eschews any comparison between himself and Mark Zuckerberg, the 23-year-old founder of Facebook who recently sold a 1.6% stake in his company to Microsoft for $240 million.

“I can’t say I never use Facebook, but it’s lame if that’s where your experience ends online,” Mr. Karp said. “As the space on the Web to identify you, it really falls short.” Mr. Karp is no fan of leading video-sharing site YouTube, either, calling it “a miserable social experience.”

This month, the New York technology blog Silicon Alley Insider called Tumblr “the darling of the New York startup scene.” Mr. Karp’s backers believe the young entrepreneur possesses an inherent understanding of where the Internet is heading.

“What makes David special is his natural feel for the Web,” a managing partner at Union Square Ventures, Fred Wilson, said. “The services he creates are very simple and elegant, yet very powerful at the same time.”

Union Square Ventures teamed up with a Boston-based venture capital firm, Spark Capital, as well as individual investors John Borthwick, Albert Wenger, Martin Varsavsky, and Jakob Lodwick to fund Tumblr for the next year or two.

Mr. Seibert and Mr. Karp’s father, Michael, an Emmy Award-winning music composer for television and film, also invested in the company.

“The VCs were ready to throw millions at us,” Mr. Karp said, adding that he declined the additional funding because he wanted to maintain majority ownership of Tumblr and leave open the option for an equity offering in the future. He said he doesn’t plan to monetize Tumblr any time soon, adding that the company’s focus is on developing the site’s features and increasing the user base.

“Our focus is not selling it to Google in two years or flipping it,” Mr. Karp said.

While advertising and charging users for premium features are ways to make money from Tumblr, Mr. Karp said he has no plans to implement either strategy over the next 18 months. Instead, he is aiming to imitate some of the magic of Facebook and MySpace, which have grown exponentially over the last few years.

“Someone of David’s age and experience knows no limits,” Mr. Wilson said. “He has the audacity to do things that someone in their 50s would never dream of. I would venture to bet he will make it happen.”