Wordpress 3.0 has merged WPMU with WP and now everyone that has a self hosted blog can make multiple websites. Many hosting providers offer multiple domain name hosting in the form of parking websites, but sometimes that is just not what you want.
Cheap hosting accounts, will usually only allow you to point domain names to your main domain, so you can't really run multiple websites. More expensive hosting accounts will allow you to park a domain, but it is usually not associated with any other websites on your host.
I like using WordPress to manage all the sites because all the websites in the WordPress install can share posts, comments, categories, etc... Not to mention, just being really easy to use WordPress