Kaxaml is a lightweight XAML editor that gives you a "split view" so you can see both your XAML and your rendered content (kind of like XamlPad but without the gigabyte of SDK).
A very handy tool to try out simple xaml; see how it look like visually without running the whole application. Very handy while working with user controls or UI generated at run time.
MoXAML Power Toys
MoXAML Power Toys is a Visual Studio Add-In that’s designed to make your coding that bit more productive for WPF/Silverlight applications.
XAML Power Toys
XAML Power Toys is a Visual Studio 2008 SP1 Multi-AppDomain Add-In that empowers WPF & Silverlight developers while working in the XAML editor. Its Line of Business form generation tools, Grid tools, DataGrid and ListView generation really shorten the XAML page layout time.
Show Me The Template is a tool for exploring the templates, be their data, control or items panel, that comes with the controls built into WPF for all 6 themes. In addition to the learning experience this provides (I'm constantly figuring stuff out about how WPF works by reading the default templates), it's also a handy way to start when you're building your own templates instead of starting from scratch (particularly for control templates).
A great utility to get hold of template of any control in any theme. Very very useful in case you need to work with look and feel of controls.
XAML Markup Styler
"XAML Markup Styler" is a xaml beautifier, which makes XAML markup source code beautify much easier by sorting the attributes based on their importance.
This tool can help you/your team maintain a better XAML coding style as well as a much better XAML readability.
A very stable and useful tool to format your XAML files easily. Look at other options for formatting XAML files here.
"Snoop" is a random utility made to simplify visual debugging of WPF applications at runtime. It's basically a collection of things that may have seemed useful at one time or another with the common goal of making it easier to track down bugs in WPF applications.
One of the most useful and must have tool for every WPF developer. It saves a lot of time in finding the UI issues in WPF aplication.
"WPF Inspector" is a utility that attaches to a running WPF application to troubleshoot common problems with layouting, databinding or styling. WPF Inspector allows you to explore a live view of the logical- and visual tree, read and edit property values of elements, watch the data context, debug triggers, trace styles and much more.
A utility similar to Snoop[still need to try this out]
Other helpful, links -
Windows Presentation Foundation Tools and Controls -
The Complete List of XAML Tools -