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.

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.

Subscribe2 Widget-hack vs Lemme Know