WordPress MU (Mutiple User) allows unlimited, separate, individual blogs to exist on one installation, in one hosting acount.

And each individual blog can have a unique domain name with the help of Donncha's Domain Mapping plugin.

The installation of the plugin requires a few additional steps.  Here's how to do it.

1.. Download the WordPress Mu domain mapping Plugin

2.  UnZip it

3.  After you unzip, you will see just three files:

domain_mapping.php

readme.txt

sunrise.php

4.  FTP to your WordPress MU site, and upload the file named "domain_mapping.php" to the mu-plugins folder.  Do not upload it to the regular plugins folder.

5.  In the host panel of your FTP program, move up a level to your wp-content folder.  Upload the file named "sunrise.php" to your wp-content folder.

6.  In the host panel of your FTP program, move up to the root level of your WordPress Installation.  Find the file named wp-config.php

7.  Use the edit fieature of your FTP program to edit the wp-config.php file online.  The only edit you need to make is to "uncomment" the  line that looks like this

"// uncomment this to enable wp-content/sunrise.php support
//define( 'SUNRISE', 'on' );

And the way you "uncomment" it is to remove the two " //" before the line that reads //define( 'SUNRISE', 'on' );

Remove the two // and save.

8.  Close your FTP program and return to your WordPress MU site.

Click Manage -> Domain Mapping

You'll know the Domain Mapping plugin is ready to use when you see this form ready to be filled in.