In a manifesto-like e-mail message sent last month to all Google employees, Jonathan Rosenberg, a senior vice president for product management, told them to commit to greater transparency and open industry standards. Rather than hoard knowledge to exploit it, he wrote in "The Meaning of Open," share it and watch Google and the entire Internet prosper.

Speaking at a conference on Dec. 11 in Mountain View, Calif., Linus Upson, engineering director at Google, said there were many discussions before allowing ad-blocking programs "because Google makes all of its money from advertising."

But he explained that the prevailing thinking was that "it's unlikely ad blockers are going to get to the level where they imperil the advertising market, because if advertising is so annoying that a large segment of the population wants to block it, then advertising should get less annoying."

"So I think the market will sort this out," he said. "At least that is the bet we made when we opened the extension gallery and didn't have any policy against ad-blockers."

As it happens, two 28-year-olds, Michael Gundlach, an independent programmer from outside Athens, Ga., and Tom Joseph, an M.D.-Ph.D. student at Mount Sinai Medical School, separately went through the exact same experience. In telephone interviews, each told of excitedly looking to see if he could install a Chrome extension of his favorite Firefox add-on, Adblock Plus, which prevents ads from appearing on Web sites, whether bright flashing animation or the text ads that Google serves up after a search.

They did not find one. So, naturally, each spent a day or so creating a rough version of such an extension, with much more work to come. AdThwart from Mr. Joseph is now No. 2 in popularity among the more than 1,200 Chrome extensions; AdBlock from Mr. Gundlach is No. 8. Together, they already have more than 120,000 users.