Here are a couple fun examples of lighting on the fly.
Both were lit with the flashlight app on smartphones.
For the top image, students held their phones to light their own faces. Not very professional, but it will do in a pinch for an informal portrait.
The second image is lit from below (see the iPhone on the table?) Again, not a professional photo, but it’s a technique you can perfect with some practice. Backlighting is great for transparent objects like glass.
My favorite way to create super portable, flexible light are LED lights. Here’s an example http://riftlabs.com/kick-videolight/