This is something that a lot of photographers, especially the beginners ones, tend to ignore, or at least underestimate. A simple white wall can become a great source of light, just bounce on it a simple speedlight, and here we have a huge diffuser, which it comes really handy when you don't have softboxes, umbrellas or any other light modifiers, but just a flash. Even with the simple sun light a white wall can become really handy if you know how to take advantage of it.
But because I'm terrible to explain things, any things, I'll just post this video from Marc Wallace, which explains how to use a white wall as a diffuser. Enjoy :-)

(via ISO1200)