hazardcell

A newer version for WP3.0 is in the works. I would have loved to release it on time for the release of 3.0 but I’m having my finals at the moment so it will have to wait until June 2010. Will include single sign-on as well.
This plugin is for WordPress MU
This plugin basically allows you to host blogs on multiple domain names while maintaining one main site.
For example, the main site could be example.com but users will also be able to sign up for blogs on example.net, example.org or any other domain name that you choose to add.
<ul>
<li>All the sites will have the same settings</li>
<li>No extra blogs/tables are created for the additional domain names</li>
<li>All blogs will be viewable on the Site Admin blog list</li>
</ul>
Alternatively, if you would like to request a feature and/or report a bug, please visit the <a href="https://wordpress.org/tags/yet-another-multi-site-manager" rel="ugc">forum</a> or the <a href="https://plugins.trac.wordpress.org/browser/yet-another-multi-site-manager/" rel="nofollow ugc">trac repository</a>. You will have to login with your WordPress.org username and password to access these.
<h3>Known Issues/Limitations</h3>
<ul>
<li>Remote login does not work with this version</li>
<li>Only tested with VHOST installs in root directory</li>
</ul>

Yet Another Multi-Site Manager

Allows blogs to be created on multiple domain names while maintaining only one main site for all domain names.

fritsjan

With this plugin you can simply batch create / add a bunch of sites to your WPMU install (WordPress Network) and use one of your existing sites as a template for the new sites. The existing blog will be cloned exactly including posts, layout, settings, etc. The plugin also takes care of domainmapping the newly created sites. (depends on ‘WordPress MU Domain Mapping’ by Donncha to be installed)
This plugin will save you a lot of time. 
Even adding one site is faster with this pluging compared to the normal way: add site, domainmap it, set everything up.
Update: You can now also use it without domainmapping, so just cloning sites in a batch! Insert the name of the subdir or subdomain you want to make in the place of the url.

Add Clone Sites for WPMU (batch)

Batch add new sites on Wordpress MU / Wordpress Network while using a template site to clone, includes batch domainmapping of the new sites.

Yet Another Multi-Site Manager vs Add Clone Sites for WPMU (batch)