locket is a Dashboard widget that provides an ambient monitor for a single iChat user on your buddy list. When the person is available, the locket opens. Otherwise, it is closed. Clicking on the open locket will start a new IM chat.
Sample lockets are free to try; locket is sold customized by the developer with your image, starting at $10.
A generic public domain version is available at the developer website. Sample shown with Joan Crawford (public domain image)