A nice little plug-in. Simple to install, easy to use.
All it does is create a page containing thumbnails of all your installed themes. Nothing else, but if that's what you need to see, this is the plug-in for you.
You can limit the page to show just a specific number themes.
You can force the width of the thumbnails to a specific number of pixels.
Anything else, I expect, may or may not appear in future releases of the plug-in.