Another vote for PHP from me! If you're at all concerned about running on OS's other than Windows (and you should be if you're talking about server software) then you should be using PHP.
Just be a bit careful about licensing. I hate to be picky but strictly speaking PHP isn't freeware. However, it does have a
very liberal license. MySQL is GPL'd, which is slightly more restrictive; if you write an application built on top of MySQL then your software also has to be GPL, or you have to buy a commercial license.
PHP can also interface to a load of other databases, not just MySQL; I know it can interface to Oracle, and it can probably talk to SQL Server too if it really has to, as well as others.
Things like phpNuke and PostNuke are content management systems (CMSs) built using PHP. You can easily use them to build and administer your site without actually getting your hands dirty on any PHP code at all. How such sites look is entirely down to how much you customise them, a lot of which can also be done without touching the PHP code at all. It's just that a lot of sites which use such CMS software simply use the default themes and a lot of other sites are so heavily customised that you don't realise they're using a common CMS!
Personally I've been using Xoops (
http://www.xoops.org/) for quite some time now after trying phpNuke, PostNuke and Mambo, and it's great!