Mass Mailer
This plugin allows you to send a mass email to your users. Originally authored by Inviz. His site sadly seems to be unavailable now. This plugin incorporates some fixes posted on the WPMU Forums by Quenting and removes "spam" from being displayed. Additionally, stripslashes() has been added to clean up the message. A call to the_quicktags() was commented out because it wasn't working on my site, so uncomment it if you want to use it (line 159). Once installed, you can access it by going to Write -> Write Email. Your users will also be able to opt out of receiving emails by going to Users -> Your Profile.