A great look and feel matters everywhere in WPF.If you had selected WPF for your project developement ,it's UI design should be stunning and excellent.
But what to do if there are no designers in your company ? Or the designers in your company don't have enough knowledge in XAML and Blend.Some time the project doesn't have enough budget to employ a designer.
Here comes the importance of ready made XAML Style packs and templates.Style packs or Xaml templates library means a collection of global Styles and templates for all standard controls which can be inserted in to your resource dictionary.Thus all your controls in that application will be in the same theme.
I just saw a site which provides these type of xaml templates.I think there will be some more companies or sites which provide ready made xaml assets.So the below list will grew as I come across new style pack providers.