Code:
# adduser webmaster
# chown webmaster:webmaster /var/www
Should do the trick.
It feels like a hack, though. Much better (IMHO) would be to have the apache `root directory' set to /home/webmaster/mysite/ or something like that. grep -re /var/www /etc/apache*/*.
Even better, for the security-minded, would be to chroot apache, run it in a user-mode-linux jail, or both. This, of course, may or may not be acceptable, depending on what the web page does.
hth --Jonas