SLCacheViewer is a Windows application that allows you to browse and export textures stored on your hard drive by your Second Life viewer. All of the major viewers store their texture cache in the same format, and this tool is designed to read that format.
In the main view, you can see all of the textures your viewer has cached.
In the image dialog, you can inspect a texture more closely.
In the sculpt dialog, you can see how a sculpt texture might look in 3D.