XAML SPY is debugging tool for WPF, similar to SNOOP but it also supports Silverlight and Windows Phone applications.
Although SNOOP is still better for debugging, XAML SPY provides some cool features like grid & ruler on live application and properties being displayed in PropertyGrid, option to download full XAML, remote debugging support, VS integration etc.
It was not free when launched but now they also have a free express version with limited features (not sure how good it will be after 21 day trial expires and it becomes express).
UI is easier to use and more intuitive, I wish attaching a WPF app. for debugging was easier and quick as in SNOOP; But it’s still useful to have this installed for some off SNOOP days.
The feature, which I liked most is saving the XAML; you can get the exact used XAML of whole window and even of a single element like TextBox. This is really helpful in knowing what XAML is created after applying all the bindings, styles, animations etc.