I haven't heard of NonProfitForce. I'll send out a Tweet to see if any of our Twitter followers have heard of how to connect this with us.
Isn't nonprofitforce the nonprofit edition of Salesforce? I think I saw several apps in the developer network section of this site.
I never received any tweets regarding nonprofitforce. But you can submit that you'd like an App for NonprofitForce with your Constant Contact account in our Marketplace.
Sorry I don't have any other helpful information.
My husband, who manages Non-Profit energy & utility cost , say's Yes! Nonprofitforce is a edition of salesforce. There is also an app called nonprofitforce. I never used it , I understand it is focused on Non-profits and their sales force.