A popular front-end is WinZip which handles ZIP files using a simple, command-line interface ZIP program but adds a windowing system with drag-and-drop capability. It also adds the ability to remove single files from a large archive, view individual files without unzipping the entire archive, and other features.
Usually, front-ends are created for an existing program. However, if a Web Browser is used as a program's front-end, as with mldonkey, the program is designed specifically for the front-end.
Front-ends are common for UNIX GUIs, where individual programs are developed on the design philosophy of many small, tested programs, able to run independantly or together.