Ask HN: Logos/licensing for gratis web app?

Our business has some spare bandwidth and time and the boss has an altruistic steak, so we built a little website to help non-profits schedule volunteers for simple events.

Because the motivation behind this website is pretty selfless, there are no ads or fees. The Creative Commons licenses don't seem to apply here, does anyone know of anything similar for "free as in beer" websites that I can stick on (logos/text) to help give it some legitimacy?

For reference, the website is http://www.volunteersignup.org/