Can I use Mac OS X server as (Web Hosting Management) software, to host websites?
Yes, you can host LAMP-based sites using Mac OS X server.
and what control panel will my clients have?
None. You will need to purchase and/or download one and install it. Since Apple has a tendency to do thing their own way with regards to the various configuration files, you may be better off using a CentOS, Red Hat, Ubuntu, Debian or, if you'd prefer, FreeBSD based distro as these tend to have much better support with the various control panels; one of the above is a requirement if you wish to purchase CPanel or Plesk. It's a similar story with other admin panels such as Virtualmin: Mac OS X is rated as a "Grade N" platform which means the installer will not work BUT if you have to ability to manually configure things correctly it should work fine.
For my personal server I run Virtualmin GPL on a CentOS 5.3 install.
