Not technically a plugin, but an add-on for WordPress that determines the last blog entry sent and mails out excerpts of all entries created since then, in neatly formatted text or HTML (multipart) email. Digests are sent to a single email address, which is most likely a separate subscribers mailing list. Format of HTML messages is fully configurable via CSS.
http://wp-plugins.net/plugin/wp-digest/
http://birdhouse.org/software/wp-digest/
spencerp