To be clear, the Rich Snippets tool is purely for testing what the formatting/markup of your content is already. It does nothing on its own.
Most of the markup that the tool is looking for is usually baked into a theme. WordPress helps this along a little bit by incorporating parts of the hAtom format into the body_class(), post_class(), and [possibly] comment_class() functions. If you browse through the code of Twenty Twelve, you’ll see that it implements more hAtom formatting with classes like entry-title and entry-content.
There may be plugins that augment those *_class() and other functions or add information to the head, but a lot of making a site “rich” is up to the theme. Plugins can mostly only control their markup (which is why the GD Star Rating plugin can make the data it stores rich but won’t help you with anything else). Yoast’s WordPress SEO plugin also can help with getting Google+ author info and opengraph data onto your site.
I often use the Advanced Custom Fields plugin to add more granular data to my WordPress sites. This lets me markup the specific pieces of data in theme files. (I wrote a blog post about marking up a People directory that the Rich Snippet tool can interpret. I’ve also done a business directory in a very similar way.) For the ACF fields, I like to use the vocabularies and markup specified by Schema.org, which is created by a consortium including Google.