Some people use mydomain.com as an example..........I am ACTUALLY talking about http://www.mydomain.com. They have a DNS service and something called stealth pointing (with subdirectory passing) that allows you to point multiple urls at one domain (in different folders). It is a free service that I have used for about 2 years now.
Maybe there is a way to do virtually the same thing by playing with httpd.conf, but I don't know how.
Yes that boils down to editing your httpd.conf to instruct your Apache server to serve the same paths and different sub-directories for each domain name. Always backup your files before tweaking them, but no danger to break anything, Apache is pretty flexible - this is basically how cheap hosting providers host several hundred domain names on one machine. (A bit schematized as there may be several web servers / virtual Apache servers implied, but you get the picture).