WordPress.org

This plugin provides a sidebar widget containing posts with the thumbnail and post excerpt (POST + THUMBNAIL + POST_EXCERPT) which have similar set of tags as the tag set of the current post

The widget displays on single pages only.

For example, if a post has the tags "tag1", "tag2", and "tag3", and there are other posts in the system with the same tags, those posts will be displayed. If there are not posts that have all three, but some with two of the three, those will be displayed. The sort order for posts with the same number of matching tags.