LinkWithin and it adds 3-5 related posts (with thumbnails, text, and links) at the end of your posts. So far it is not perfect, as the related posts that have been chosen do not always relate, but I don't use a large variety of keywords either. I do think the widget will be useful to keep readers interested in my blog.
Right now LinkWithin is free and advertisement free. It works with Blogger, WordPress, and TypePad, and there is an option for other blog platforms, too.
Strange thing I also noticed after posting this is that LinkWithin has chosen some external posts for this particular article. I wonder if this is because I am talking about their widget? All of my other posts have links to my own blog's posts- so I will keep an eye on this as I am not interested in advertising other sites.