The aha moment that led to Trello, says its creator Joel Spolsky, was a series of "customer visits to see how people were using Excel."
His findings, gleaned while Spolsky was a program manager on Microsoft Excel team in the early '90s, won't surprise you. People didn't do much calculation in their spreadsheets, but they did a whole lot of 2D layout. Excel was, and remains, a great example of a user innovation toolkit -- a product that's malleable enough to enable its users to reshape it according to their needs.