The GyrumSimpleSideMenu module (used at left on this site) is a simple tree-like menu system. It has options to change the starting point of the menu, to change how many levels to display, and to display page titles or names. It also has some other stuff. If enough people want it, I can add CSS support so that different levels can have different display styles.
For now, it's just a simple menu that comes in handy for certain scenarios. It's free, so feel free to download it and see if it will work out for you.