Do you have projects where you can't predict an end date? These tend to be a job search, a change project, and with a tip of the hat to Cesar Abeid, your life. I like to call these “emergent” projects.
You might prefer to call them “adaptable” projects, but to me, every project has to be adaptable. These projects are emergent. You need to plan, but not too much. You need to replan. You need to take advantage of serendipity.
My column this quarter for projectmanagement.com is Applying Agile to Emergent Projects. (Free registration required.)