Backstory: I volunteer for a few local community organizations and around the holidays I always think about finding another. The trouble is that unless I have a specific type of work in mind, I don't know what's out there.
The Idea: A collection of nonprofits organized by category, location and rating.
Deep Dive: You would build a website for people to submit their nonprofit organization. Users would be able to assign categories, location and various other details about their nonprofit. Volunteers of these non profits would be able to create an account to rate the nonprofit.
The idea is to crowdsource this information but to start you would have to manually input a few. You would get the best traction focusing on a specific city and doing local advertising there.
Validation: I've been to local conventions/markets where only nonprofits are allow to attend in order to find/recruit more volunteers. Starting in a specific city will help gauge user interest.
Monetization: You could have target ads but there may be an opportunity to get grant money or turn this into a nonprofit itself to help raise funds.