There is Expression Blend, Visual Studio, XamlPad that we are all aware of and use on a regular biases for creating our XAML. One that I am adding to the top of my XAML editor list is Kaxaml! This is the lightest weight most useful XAML editor I have ever seen! It's got a great WPF/Blend look and is light and fast. No more waiting 10-30 seconds for Visual Studio to fail to load your XAML documents! If there were a way to replace the Visual Studio editor with this I would totally do it!
Unlike Blend it has code insight. Unlike Visual Studio 2008 it is fast and functional! My favorite feature is the XAML Scrubber which will "pretty up" your XAML by organizing the tags and attributes. It will even put the most important attributes at the start of the tag. It removes much of the pressure of writing XAML!
Hopefully some future versions will include a visual tree and perhaps even a feature where you can click on an click on an object and be taken to the applicable part in the XAML. The XAML Scrubber only uses spaces and I am a tab guy -- oh how I wish it supported tabs! Also, I noticed that once I added an event to a control it said I had to compile the XAML -- not sure what that means. That means it's probably mostly useful for creating the user interface before you start hooking code into it.
If you need a good place to hammer out some design -- this may be your best bet!