Over the past few years we've seen Chrome plugins that let you browse Amazon and check books out of your local library, borrow books on Oyster's (now defunct) ebook service, or reverse showroom and buy books in either the UK or France.
Mockingbird works a lot like the other Chrome plugins mentioned above.
Simply browse the book category or the Kindle Store on Amazon, and the plugin will search Project Gutenberg and tell you whether there is a matching title on the PG website. It will prompt you with links you can click to visit PG and download either the ebook or the audiobook (I'd pass on the latter; the TTS voice isn't any good).
If you find a book, you can download it for free in a number of formats, including Kindle, Epub, PDF, etc. You can also read it on the PG website (this plugin might help you there).
But depending on the title, you might not want to. I checked a half-dozen titles in the Kindle Store and found that all of the public domain works were already available for free on Amazon.com (in what are probably nicer looking ebooks).
The Project Gutenberg integration is really just the beginning of what can be down with this plugin.
What would you like to do with it?