There is a xamlviewer that is part of the SDK. If you install Avalon, it is located in Program Files\Microsoft SDKs\WinFX\bin\xamlpad.exe
ChrisAn wrote XAMLPad and then has provided an updated build on his blog called AvPad, with enhanced features:
- Ships with a complete custom style for all Avalon controls
- New "radial" menus animate in from the corner (ok, this is kinda a hacked in feature <G>)
- Better background parsing (waits to parse based upon the duration of time it takes to parse)
- Child window manager implementation
- Visual tree viewer
So, his new one is probably the recommended choice. http://blog.simplegeek.com is the place to go.