Maybe it's in tabs? Or maybe something else?
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This sort of depends on how you define "popular", but you can search for all posts with a particular tag (or combination of tags) and then sort them in various ways.
For example, you can search for all the questions with the c# tag just by clicking on the tag wherever you see it on the site.
And from the results page (which shows, at the time of this writing, nearly 200,000 questions with that tag), you can choose to sort those questions any of 6 different ways: newest, featured, faq, votes, active, and unanswered. These are the tabs at the top of the page. Click on one to change the sort order.
I'd say that the most popular questions are probably those with the highest number of votes, so you can click the "votes" tab to sort by the number of votes.
And if you really want to get high-tech, you can visit our advanced search page and search by whatever parameters you like. All of the available options are detailed on that page.
Warning: Be careful with this type of search, though! You're likely to find a lot of old questions that, while popular, no longer fit our guidelines for acceptable questions. Our rules have changed and evolved a lot since the early days of the site, when dinosaurs roamed Stack Overflow.
Since people find these questions useful, we haven't been able to bring ourselves to delete them from the site. But please be aware that they exist only as a historical curiosity; their presence does not provide sufficient justification to ask such types of questions yourself.
You can think of them as being effectively "grandfathered in".
For more information, see this question: Why do some off-topic questions get closed, and some don't?