I love shooting portraits on location and get hired a lot to photograph people for corporations or magazines. While I like being able to set up multiple strobes and spend the time really getting my lighting perfect, I'm more often under some extremely small time limit. For those "speed portraits", I'll often use an on-camera flash but move it off camera. The speed light becomes my key light source and I can use a second light as a background fill light. Couple this two-light setup with the ambient light and it makes for a nice portrait.
I had about 5 minutes to shoot with this busy pharmacist; using a simple speed light setup allowed me to get the shot. ISO 320, 1/50 sec. at f4, 50mm lens.