Salesforce provides 10 free Enterprise Edition Licenses of Salesforce Sales Cloud - CRM to Non Profit Organizations, Higher Education and  K-12 Education based organizations. 

Non Profit Organizations can make use of free NPSP (Nonprofits Success Pack)
Higher Education based Organizations get free HEDA (Higher Ed Data Architecture)

First of all, you need to find out of if you are eligible to receive the Salesforce free licenses. You can check your eligibility here. Once you confirm your eligibility, head straight to Salesforce.org and sign up for the Free Trial. The free trial needs to be taken first in order to get access to 10 Salesforce free licenses.

In the Salesforce.org page click on “Get Started”
Scroll down to the “Salesforce for Nonprofits” section and click on “NPSP Free Trial”.