Using nrelate for my newest project and it's by far my favorite plugin for displaying related post. The default templates look very nice and I like how it can take the first image used in a post and use it for the thumbnail. Very easy to configure and since it is served by a third party it will not have negative impacts on smaller hosting accounts. It does take a bit to start displaying related posts (around 15 minutes or so) as it first has to index your site. Not a big deal for me. Highly recommended.