Give – Donation Plugin – this plugin works great for pushing donations to Paypal and for suggesting giving amounts for a donor to consider
If you are doing giving through Authorize.net or Stripe you can set it up through Gravity Forms.
Gravity Forms for all things form related. Gravity Forms also stores the form submission data in the database so that the nonprofit can review it later
The Events Calendar is fantastic
Photos (Minimizing Photos Filesize)
WP Smush will help to make sure your site is not overloaded with big images.
If you are working with a nonprofit that is, for example, a shelter for battered women, those women cannot be found on a website for a shelter for battered women. This plugin makes is so that when someone is on the website, if they press the Escape Key on their keyboard it instantly takes the user to Google, away from the website.