I prefer to use the available light when possible, and love the natural look it gives to portraits. I will occasionally use another light source if necessary, or if I'm going for a certain look with the portrait that requires an extra light source.
I took these goofy portraits of Katy at Bethabara when I was interning with Claudia Page at The Portrait Gallery.
Back-lighting in portraits is one of my favorite ways to use the natural light. The light on Lizzy's face really makes her profile pop against the background.
Side-light gives a more dramatic look to portraits, like in this portrait of Luke at around 8 months old.