For all those web developers who rushed out to upgrade to OS X Leopard - if you're like me and you used the Sites folder in your home directory for testing your websites, you may have noticed that you can no longer access http://localhost/~username/ after your upgrade to Leopard. It took me a minute or two to figure out, but here's the problem and a solution (if you are command-line friendly):
 
Leopard features the Apache v2 webserver, as opposed to the older Apache v1 in Tiger. In the old Tiger OS, the Apache configuration files were stored in /private/etc/httpd/. That's changed in Leopard. The new path is /private/etc/apache2/.
 
When a user is created on your system, a small Apache configuration file is created that enables Apache to serve content from their ~/Sites/ directory. Under Tiger, these files were stored in /private/etc/httpd/users/. From what I can tell, if you've done an upgrade from Tiger to Leopard, those files do not get migrated over to the new /private/etc/apache2/users/ folder.
 
So, in order to make your sites work again, make sure to copy your Apache per-user configuration files from /private/etc/httpd/users to /private/etc/apache2/users. For example, I had to do this:
 
$ sudo cp /private/etc/httpd/users/michealk.conf /private/etc/apache2/users/
$ sudo apachectl restart