Thumbnail For Excerpts allow easily, without any further work, to add thumbnails wherever you show excerpts (archive page, feed...).
The idea of this plugin is to make excerpts have an image near. By default, WordPress strip every tag in excerpts, including images, so the look can be boring. Since WordPress 2.9 there is a mechanism for thumbnails, which still ask for some coding. Not with this plugin! With this plugin your life is easier! Keep in mind that this plugin use the new feature from WordPress 2.9, but works with previous versions too. It will use any specific thumbnail you have indicated manually. In the same time, it will provide automatically thumbnails for all the posts (previous posts for example) which have at least one image, but no manually-indicated thumbnail.