If you've used XamlPad from Microsoft ... you probably have a love-hate relationship with it. You love it because it's reasonably quick at doing a proof of concept or test of a WPF feature. You love it because it's simple. You hate it because it has a bunch of odd quirks with the text editor, the fact that it really has only a few useful features ...

Kaxaml is a better XamlPad in many respects. It's got a bunch of nice little features including:

• Intellisense!
• A color picker (that is better than the one in Expression!)
• a scrubber:
• 
• You can paste images into the editor!
• It can have multiple files open at one time
• Did I mention intellisense (and a color picker!)

The full announcement is here.

Pay attention to the default install -- it may change your default file handlers for Xaml files in a way that isn't to your liking:

A definite recommended add to your WPF/Xaml editing toolbox.