notification-daemon displays those "you've got 10 minutes of battery left" dialogs. To launch it, you send a dbus signal. A file in /usr/share/dbus-1/services launches the servicce when the dbus interface is called.
An interesting thing I noticed today: if I kill the notification-daemon process, I can still get messages. The process just gets relaunched by dbus. Why the hell doesn't the process quit when no notifications are active? This would save 3.1 MB of memory on my system.