Fill in flash. Outdoors in daylight you can use flash to add a catchlight to eyes so they don't look dead but have a sparkle.
Fill in flash. For portraits on a sunny day you can use flash to remove harsh shadows, maybe from a hat. Just force on the flash.
Create a black background in daylight by manually under exposing to produce a dark or black picture then use flash to light the subject.
http://www.botzilla.com/photo/strobeVolts.html
