The WPF team has a home on CodePlex where they release the WPF Toolkit and other goodies. To quote their project description,
The WPF Toolkit is a collection of WPF features and components that are being made available outside of the normal .NET Framework ship cycle. The WPF Toolkit not only allows users to get new functionality more quickly, but allows an efficient means for giving feedback to the product team. Many of the features will be released with full source code as well.
One of the nice things they have added is a set of nine lovely WPF themes you can use in your applications today. Use the link to download them and to see the instructions for theming your app.
The appearance changes dramatically - with no coding on your part once you've hooked in the theme!