Martha Alderson, a.k.a. the Plot Whisperer, sent out a newsletter with this useful list of important scenes:
1) Set-up: The set-up you create in the Beginning makes the journey the protagonist undertakes in the Middle feel inevitable.
2) Inciting Incident: A moment, conflict, dilemma, loss, fear, etc. that forces the protagonist to take immediate action.
3) End of the Beginning: The protagonist's goal shifts or takes on greater meaning and turns the story in a new direction, launching the character into the actual story world itself.
4) Halfway Point: The moment the protagonist consciously makes a total commitment to achieving her goal and does something that signifies she has burned all bridges back and thus can only go forward.
5) Crisis: The all-is-lost moment.
6) Climax: Just as it looks as if all is permanently lost for the protagonist, she saves the day.
Meanwhile, Jenny Crusie offers a chart to help us visualize plot. It's on her blog.