You can add a Google+ community, collection, or profile stream to your site.
Google+ has been shut down so this information is redundant.
On your computer, open Google+. Go to the Google+ page of the stream you want to add to your site. Copy the URL of the Google+ page.
Open a site in new Google Sites. At the right, click Embed. Paste the URL of the stream.
Click Insert. At the top right, click Publish.
Note: You can add a top-level community stream or specific community category streams to your site.
This may be rolling out and not yet available to all.
Note (from Steegle) You can see a demo of it at Insert Google+ Content Streams- new Google Sites. BE AWARE that the Google+ streams are only visible on the sites.google.com address and will not be visible on the Custom URL address.