Google provides some basic gadgets that allow users to interact with a site: the members gadget that lists some of the people who joined the site, the wall gadget that lets you post general comments about the site and the review gadget, which associates the comments with an item from site (a blog post, a video). Friend Connect also supports OpenSocial gadgets, but there's no official directory of gadgets.
Google's new service is not yet a good way to build a community because it doesn't integrate with a site. You can only customize the appearance of a gadget, but you can't integrate your existing commenting system with Friend Connect or gain access to in-depth stats about your members.
You need to upload two files for RPC relay and canvas view, which is not always possible if you use a hosted blog service. Fortunately, Blogger already has the two files, so it's even easier to setup Friend Connect.
I added the members gadget and the wall, where you can post comments and links to YouTube videos:
Here's a short video that explains how to use Friend Connect: