A project timeline is a valuable thing to produce during a project retrospective. A timeline should show the various events that occurred during the project, and how they affected the project.
At XPday, Tim Mackinnon showed a quick and effective way to create a timeline. First mark out an area on the wall and mark it off with rough dates (use a whiteboard or paper attached to the wall). Then each member of the team puts stickies on the wall to show events. These events are color coded: green for good events, orange for problematic events, and yellow for neutral events. Once everyone has put events on there you then do a second pass. With the second pass people put up stickies that show how they feel: pink for happy feelings, blue for low and frustrated feelings.
Once you have a timeline you can use the timeline to trigger discussion about the project and mine the timeline for lessons learned (good things to repeat, things to do differently, things that are puzzles, and actions for the future.) It's particularly interesting to look at the timelines for such things as good events with lots of blue feelings - a surprisingly common case.