Viktor Szépe

Enables users with <code>list_users</code> capability (Editors and Administrators) 
to send plain text message to any user in an email.
There is a new action “Send message” provided on the Users admin page 
for each user.

Send message

Send an email (subject and text message) to WordPress users.

Abhishek Kumar

This plugin allows you to send html new user notification emails in WordPress.
It provides wp editor to edit your email template.
You can also upload images to your mail content.
You can change sender’s name and email.
This plugin was created by Abhishek Kumar http://github.com/abhishekfdd

Html New User Notification

This plugin gives you option for sending html new user notification email.

Send message vs Html New User Notification