Stack Overflow is primarily a message board where programmers can share tips and tricks and crowdsource solutions to problems they run into while coding. The name is a reference to a programming error.

Stack Overflow grew from seven million visitors to over 16 million in 2010, putting the site in Quantcast's top 400, CEO Joel Spolsky wrote in a blog post. The company raised $6 million from Union Square Ventures and angel investors last spring and went from three full-time employees to 27.

It's been so successful that the company launched several new sites in 2010, including a Q&A site for cooking, math, photography and English.

Over 80 percent of questions get a good answer, Mr. Spolsky wrote, and many of the new Stack Exchange sites have 100 percent answer rates. One of the issues with Quora, a well-funded Bay Area startup founded by former Facebook employees, is the high number of unanswered questions.

One way to solve this problem, Mr. Spolsky realized, was to separate Q&A sites by topic--that way each site attracts a more engaged group.

"We learned a long time ago that the only way to get questions answered promptly is to have a critical mass of knowledgeable users, so we have an onerous process called Area 51 where sites are proposed, discussed, and voted on. If a proposed site doesn't have critical mass, we just won't create it. Even if it does get created, it has to maintain a certain level of traffic and quality or we'll close it down," he said.

Another New York company has had success by targeting developers--Forrst, which is like a Tumblr for programmers, has been growing rapidly since founder Kyle Bragger started it as a side project.

UPDATE: Mr. Spolsky pointed out on the tech news forum Hacker News that Quora and Stack Overflow are targeting different kinds of questions. "We're the reference section of the library, they're an awesome salon where smart people are shootin' the shit," he wrote. The questions on Stack Overflow and its associated sites are the sort of questions that have definite answers, like "Is there a word or phrase for the feeling you get after looking at a word for too long?" from the English Language & Usage site (answer: semantic satiation. Those questions can be found on Quora, too, but it also attracts questions with subjective answers, i.e. "What tech company will we all be talking about at SXSW 2011?"

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