Some people don't believe in a CMS, but given how cheap hosting is these days, I would recommend getting one setup and learning how to configure it over a static HTML website. So if you want something more customizable:
Wordpress (
http://www.wordpress.org). It's setup to be a "blog" platform, but honestly you can do just about anything you want with it. Lots of ready made themes out there that you can learn from, plus the PHP coding is of pretty decent quality if you care to learn any of that. The documentation is very helpful (something not always found in the open source world).
The nice part is that it will mostly handle basic search engine optimization for you, which can be important for a business oriented site.
If you're really brave, Drupal (
http://www.drupal.org) is pretty much the best open source CMS platform out there right now. It can be customized to do absolutely anything, but it comes with a massive learning curve. I've put nearly 6 months of time into learning the software and I still feel like I'm drowining in it. Still, haven't seen anything that really competes with it's flexibility (if anyone knows of something similar, let me know!).