Josh

This plugin will allow you to add a custom welcome message to your login/register screens. It will also allow you to add a separate custom message for your logout screen. The admin page allows you to add HTML code into the message boxes, so you can include images, links, videos, or any other HTML code. The options are only limited by your imagination. Check out the screenshots for an example.
Features:
<ul>
<li>Add a custom welcome message to all users who visit your login/register page.</li>
<li>Add a separate custom message to your logout page.</li>
<li>Change the default wordpress logo to your own custom image.</li>
<li>Change the default hover text from “Powered by WordPress” to your own custom message.</li>
<li>Change the default logo link from “www.wordpress.org” to your own custom link.</li>
<li>Manipulate the CSS for the message boxes to make beautiful effects and custom styles.</li>
<li>Add links, images, videos, embeds, iframes, or any other HTML compliant tags.</li>
</ul>
Support: 
<a href="http://forum.joshlobe.com/member.php?action=register&referrer=1" rel="nofollow ugc">Visit my support forum.</a> 
<a href="http://www.joshlobe.com/2012/01/wordpress-custom-welcome-messages/" rel="nofollow ugc">Visit my personal blog page for this plugin.</a>
Other Plugins by Me: 
<a href="https://wordpress.org/extend/plugins/ultimate-tinymce/" rel="ugc">Ultimate TinyMCE</a> – Finally unlock the true power of your visual editor.
<h3>Features</h3>
<ul>
<li>Add a custom welcome message to all users who visit your login/register page.</li>
<li>Add a separate custom message to your logout page.</li>
<li>Change the default wordpress logo to your own custom image.</li>
<li>Change the default hover text from “Powered by WordPress” to your own custom message.</li>
<li>Change the default logo link from “www.wordpress.org” to your own custom link.</li>
<li>Manipulate the CSS for the message boxes to make beautiful effects and custom styles.</li>
<li>Add links, images, videos, embeds, iframes, or any other HTML compliant tags.</li>
</ul>

Custom Welcome Messages

This plugin will allow you to add a custom welcome message to your login/register screens.  It will also allow you to add a separate custom message fo …

ymjid

Webinfos allows to admins to show a customisable message in the dashboard in is own dashboard widget. The plugin supports several custom messages (only one will be visible on the dashboard). The plugin is translatable but only French is supported for now. Webinfos integrate a feedback form to report a bug and/or give suggestions to the plugin author.
With Webinfos you can :
<ul>
<li>Add Contact informations (photo, phone number, email) and choose if you show them or not.</li>
<li>Upload logo, video or just files as attachments.</li>
<li>Use HTML tags in the message.</li>
</ul>

Webinfos

Webinfos allows to admins to show a customisable message in the dashboard.

Custom Welcome Messages vs Webinfos