I have always used StyleSnooper to get at the templates for my WPF controls. It's pretty useful and simple, and can get you the style and templates.
Now I just stumbled into a new tool, by Chris Sells and Ian Griffiths, called ShowMeTheTemplate.
This is nice. It formats the output a lot better than StyleSnooper (you can expand and collapse ala IE) and it splits the different templates (e.g HeaderTemplate and ContentTemplate into separate text boxes). You also get to see a preview of the control you are viewing the template for. Finally, you get to choose which Theme to use.
This should prove a useful addition to the WPF developer's toolkit especially combined with StyleSnooper for seeing all the other styles applied by default to a control.