Suggestions for a voting/poll widget that's not easily spammed?

I have a client who is going to be letting their customers vote on what non-profit to donate a percentage of their revenues to. I want to install a voting mechanism for them that at least makes it slightly difficult to spam it with millions of fraudulent votes. :)

Anybody have any recommendations for a 3rd party polling/voting mechanism that I can leverage that would fit the bill?

Thanks!