Salesforce provides 10 free licenses of their enterprise software to nonprofits under the Power of Us program. Nonprofits must apply to receive these licenses. If you are working as a consultant for a nonprofit that does not yet have licenses, you must sign them up.
Once you sign up for a trial org and go through the application process, your instance of Salesforce will come with NPSP already installed.
Important:When you want to learn NPSP, you must set up a Trailhead Playground first and then install NPSP at a special page meant just for Trailhead.
Whether you have a production version of NPSP through a nonprofit or are working with a trailhead playground, additional packages can be installed to extend functionality.
Free add-ons include:
Program Management
Outbound Funds Module
Volunteers for Salesforce
You can find all the add-on apps, both paid and free,