Changes

The Contact Merge page has been updated to use Lightning styles.
The Account Model Conversion Utility page has been updated to use Lightning styles.
In-Kind Gift support has been added to NPSP. Additions include 4 new Opportunity fields, 1 Opportunity Page Layout, 1 Workflow Rule, and 1 Field Update. Please see the provided documentation to enable this feature and configure your system to support In-Kind Gifts.

Issues Closed

#1940: unexpected results for a 3-way Contact merge
#1950: NPSP Settings for Address: Settings not saved: Attempt to de-reference a null object.

You can use the link below to directly install 3.66 if you are already running on a recent version of the NPSP:
https://login.salesforce.com/packaging/installPackage.apexp?p0=04ti00000011Mw5

If you are running on a 2.x version, you should upgrade to the new release using the installer:
https://mrbelvedere.salesforcefoundation.org/mpinstaller/npsp

The Trialforce Template ID for this release will be posted here when it is available.