Save an Excerpt Automatically When One Is Missing
Fills in the excerpt field from the first sentences of the post when it is saved without one, so feeds, plugins and APIs that read the stored excerpt always get a summary.
Snippet Library
Content snippets for WordPress
Snippets that add to or change your posts and pages: reading time, author boxes, “last updated” notes, content after the post or after a paragraph, and excerpt tweaks. For bloggers, publishers and anyone who wants richer posts without editing theme files.