Thumbnail For Excerpts allow easily, without any further work, to add thumbnails wherever you show excerpts (archive page, feed...).
I've been using this plugin for few years and it never fails.
I am using WP 3.6. I am glad that I found this plugin and that it worked immediately. No doubt, excerpts look much better now. As I have read, the plugin was last updated a few years ago. Great performance !
Compatible up to 3.5.1