WordPress 3.0 offers a lot of new features, one of my favourite are multisite option where you can create different website on a single domain name. This feature is basically from WPMU but eventually incorporated in WordPress, the feature is called Network or Multisite option.
Also if you’re a theme creator this feature can really help you when creating custom themes, for demo and different setups in one installation.
By default this is disabled, if you want to enable it follow the instructions below.
If you want to know how to do this on localhost, I’ve posted an article how to install wordpress on localhost.
First go to your wordpress root folder, open you wp-config.php. Add the following line below.
Once your finished adding the code, save it and go to your wp-admin dashboard and browse to Tools>Network.
Note: If WordPress ask you to create a blogs.dir do not create a .php file, instead create a folder and name it blogs.dir in the wp-content folder.
To create a new website using this feature, go to network admin (on the right-top beside log out) . From there go to Dashboard>Add site, and enter your preferred name. You can browse the newly created site by putting this on your address bar http://NameOfsite.OldNameOfSite.com/
Now you can run multiple wordpress website, you can also visit wordpress codex for the official documentation.
Hope this simple tip will be able to help you.