SoloMail is a WordPress plugin designed to email an HTML-formatted copy of single posts (not batch digests) to site subscribers. There are quite a few solutions out there designed to send regular digests of recent posts to subscribers. SoloMail solves a different problem – you may want to cherry pick just certain posts to go out via email, and you may want the ability to “send now.”
The subscriber list can EITHER be the group of all registered users on a site OR a single address, such as a mailing list. Though you can use SoloMail either way, I recommend sending through a proper mailing list, which provides advantages such as handling and unsubscribing bounced/dead addresses, and metered sending for large lists.
Please see the included readme.txt for configuration and usage instructions.