Some people ask questions here out of simple curiosity. But some ask them because they are working on a research project, come up with a question they need to know the answer to, and think that the answer is probably known. In the past, one had to search for the right person to tell you the answer or trawl through lots of books and articles, often not knowing quite where to look. Mathoverflow ought to be a far more efficient way of doing it.

So my question here (asked out of simple curiosity) is whether there are some good examples of people using mathoverflow in an essential way to solve a research problem. The best example would be a story such as this: you struggled to solve a problem, you identified a statement that you thought would be helpful, you asked about it on mathoverflow, you got an answer, and the answer was just what you needed to complete your research project.