Themes for WPF
Themes is a great way to customize look and feel of your application and an easy one too. You can customize them or design your own but designing your own theme requires a lot of expertise and time. From my point of view its not a great idea to invest in designing your own theme provided you have so many themes available for you. You can take one which suits you and modify it as per your needs.
So I decided to post various links I know for this purpose. Let me know If you know any other 
Open Source
Default WPF Themes:
The desktop themes determine which resource dictionary is used. To get the resource dictionaries for the desktop themes, see
WPF Themes:
WPF Themes is a collection of free themes to be used in WPF projects
http://wpfthemes.codeplex.com/
WPF Futures Themes:
WPF Futures includes sample controls and features, many of which are being considered for the Toolkit. It contains an updated collection of seven themes replaces the original nine WPF Themes.
http://wpf.codeplex.com/wikipage?title=WPF%20Themes&ProjectName=wpf
Others:
WpfStyles.com lets you download, upload and share your WPF and Silverlight components - all in the one place, for free, and no fuss.
(Update: Unfortunately wpfstyles.com is not hosted anymore :( I hope the themes there were backed up somewhere!)
3rd Party:
XAMLTemplates.net
http://www.xamltemplates.net/wpf-themes/
Reuxables: [Some free themes are also available]
http://www.nukeation.com/reuxables.aspx
WPFreakyStyley
Other Resources:
WPF Theme Selector:
This Visual Studio 2008 Add In makes it easy to apply a theme to your WPF project.
http://wpfthemeselector.codeplex.com/
theMatic–WPF Theme Generation Tool:
Doing a custom control theme in WPF is no small undertaking. theMatic (currently very-much-in-alpha) hopes to take some of that pain away, especially for smaller teams that don’t have a dedicated designer, or for quick apps that you still want to give that ‘custom’ touch.
Saha Wpf Theme 2012/2013
Wpf Project With 21 Themes
https://marketplace.visualstudio.com/items?itemName=AliAlikhani.SahaWpfTheme2012