Rafal Toborek

This plugin is currently in alpha stage. It includes only basic features like sending e-mail notifications using built-in PHP mail() function or by using external SMTP server (recommended). Work of this plugin depends mostly on the SMTP server configuration.
Lemme Know plugin allows you to send e-mail notifications only for a small amount of subscribers. There are plans to implement Cron-based solution which will allows to send notifications in chunks and omit server limitations.

Lemme Know

Sends e-mail notification for all subscribers when a new post is published.

klitscher

This plugin is really just a widget to go along with the Subscribe2 plugin. There is a conflict with some plugins and the widget that comes with the Subscribe2 plugin due to the use of the-content(). See <a href="http://subscribe2.wordpress.com/2006/11/16/widget-conflict/" rel="nofollow ugc">here</a>. This widget does not use the-content(), but functions in the same exact way as the original widget.

Subscribe2 Widget-hack

Creates a sidebar widget for the Subscribe2 plugin if you have other plugins that are incompatible with the original Subscribe2 widget.

Lemme Know vs Subscribe2 Widget-hack