Sites such as GreatNonprofits (http://greatnonprofits.org/), Guidestar (www2.guidestar.org/Home.aspx) and even Yelp (www.yelp.com) allow people who aren't directly paid by a nonprofit -- such as donors, volunteers, board members, and customers or recipients of services -- to post their opinion. And as always, the complainers often seem to be the loudest, or the most motivated to vent.
Take a look, and don't freak out if you see any negative comments. It's as an opportunity to fix things before they bother someone else. But also get the word out to the people who know and love your work that you could use some reviews from them -- honest ones, but hopefully also bringing attention to the best of what you do. It will take them a mere few minutes for each one. That's a lot of marketing bang for the buck.