The majority of sites I build are based on WordPress – not only because it is free, amazingly extensible (is that a real term?) and just plain cool.. but I also really like it because ANYONE can use it. While there are tons of great themes out there to be had for free, it is often hard to find a really nice one to run a business site on. I have an answer for that issue though – ThemeForest (also on twitter).
ThemeForest has a fantastic selection of “premium” themes – most of which can be had for under $30. There are many other “premium theme” sites out there, but it is hard to trust many of those sites. ThemeForest is a solid choice – and clearly run by some smart people (Envato).
A sister site of ThemeForest is GraphicRiver – piles of fantastic artwork for dirt cheap. Good stuff!