NativeViewer is a Visual Studio extension for visualization of OpenCV images stored in the cv::Mat class. A thumbnail image is shown during the debug when a variable is hovered by mouse with CTRL key pressed down. NativeViewer also formats the string displayed in the "Watch" window to express valuable cv::Mat information.
The extension works only for native C++ code for both Win32 and x64 platforms. All Visual Studio .NET versions are supported. However, installation process differs for versions 2012, 2010 and pre-2010. Check the quick start guide for details: https://sourceforge.net/p/nativeviewer/wiki/QuickStartGuide
Note that if you have Visual Studio 2012 or later, I recommend using Image Watch extension from Microsoft: https://visualstudiogallery.msdn.microsoft.com/e682d542-7ef3-402c-b857-bbfba714f78d