Gokul Rajaram, a high-ranking Google product manager who helped launch one of the search giant’s most profitable ventures has left to start his own company.

“I’ve been having the itch to do something entrepreneurial for awhile now,” says Rajaram, 33, known by Googlers as one of the “godfathers of AdSense.” “I’m in my early 30s and I have some experience and financial security, so the time felt right.”

“I have some ideas on the consumer Internet side,” he adds. “I’m still trying to flesh that out in the next few weeks. Right now, I’m just trying to get used to not going to work.”

Rajaram, who left Google on Nov. 2., says he’s also interested in pursuing journalism or writing.

Whatever venture he pursues, he has the blessings of top Google (GOOG) brass Eric Schmidt and Sergey Brin. Product management vice president Susan Wojcicki honored him with a large plaque signed by a few hundred Googlers at his farewell party at the Googleplex in Mountain View, Calif., two weeks ago.

When Rajaram joined the company in January 2003, Brin and Wojcicki were cooking up an idea to sell advertising space to run relevant ads on publishers’ websites. They saw huge growth potential for AdSense based on the early success the year before of AdWords, which allows advertisers to bid for prime real estate on Google’s site whenever a person performs keyword searches.

AdSense has revolutionized web publishing by turning blogs and content sites into profitable businesses. In the third quarter, Google generated $1.45 billion from AdSense, a third of its revenue.

“It was a high level concept back then,” Rajaram says. “Google had already understood how to match keywords with URLs, so the next step was building a matching algorithm and create an advertising system around it. Sergey had a mandate to launch it in June 2003. So we had less than six months.”

Rajaram considered staying on to handle the integration of DoubleClick, the display-advertising network that Google announced in April it was acquiring for $3.1 billion. But he says he misses working on a small team and he hopes to recreate the startup environment when he launches his own company. “When we started AdSense, it was just me and four engineers,” Rajaram says. “The night before we launched, Sergey spent five hours with me testing the system and pointing out bugs.”

Rajaram joins a number of top Googlers who have departed recently, including former e-commerce director Benjamin Ling and ex-YouTube CFO Gideon Yu who both joined Facebook.