There are a number of plugins out there designed to scan a WordPress site on a periodic basis (e.g. nightly), grab all the recent posts, and tidy them up into an email digest. Heck, I even wrote one of my own a few years ago. Some work as WP plugins, others scrape RSS feeds.
But none of them let you hand-pick the posts you want to send by email, none of them let you “send now” and few of them provide good controls for managing the HTML/CSS of the email template. So I decided to write my own. SoloMail uses the excellent PHPMailer class, which is now included in WordPress core, and provides a simple checkbox on post views that lets you “Send now.” The current post is wrapped in a completely customizable HTML email template, and sent either to all registered users of the current site or to an external mailing list (preferred).
To see it in action, subscribe to Birdhouse Updates.