We introduced a new feature today that allows you to visually group your active projects by using separators. Keeping your projects on a single flat list is critical to the GTD workflow. This new feature is intended to let you maintain that simplicity while allowing you to visually organize them.
Click the new "Add a Separator" link at the top left of your project view and give it a try! You can name them, drag them around, and collapse them to hide projects between them.