One of my complaints about Amazon streaming for Prime Members is that they don't make it easy to search and browse only within the stuff you can get for free. This seems to be the case with their new Kindle Lending Library as well. Here's a recipe for browsing the list (currently at 5,379 results) from this Amazon discussion thread:
Follow these steps to browse books that are in the new Kindle lending Library on your PC.
1) When on the front page of Amazon take a look at the search function near the top of the page.
2) Don't put anything in the search box. Select "Books" in the department drop down box. Click on the "Go" button.
3) You will now see pretty much all of Amazon's books. All 34+ Million of them. Select "Kindle Edition" in the Formats you see up at the top of the results list.
4) Now you have all 1+ Million Kindle books. Over on the left side of the screen is further filters. Go all the way near the bottom of the long list of filters is a check box for "Prime Eligable". Click on it.
5) There ya go. All 5,377 Kindle books that are in the new Prime Lending Library. This is all of them as that is the same number of books that I got when I was looking at them on my Kindle.
Apparently this only works if you're already a Prime member. It also appears as if you then need to use your Kindle device (the lending library only works with Kindle hardware--not the apps for devices like the iPad) to actually download the title.