by Josh Hartman on WordPress.org
Require users to add at least one post tag before saving a draft, updating a post, or publishing a post. This applies to normal posts and may not be …

Alert appears when you try to save a post without adding at least one tag
Tired of untagged posts? Use this simple plugin to require users to add at least one post tag before saving a draft, updating a post, or publishing a post. This applies to normal posts and may not be compatible with custom write panels or custom post types.