Sidebar Recursive Slideshow
Say you have your images in a folder named A, and inside folder A there is folder B, C and D which include more pictures. If you configure the Image Slide Show sidebar gadget to display pictures from folder A, including its subfolders, and you enable the "shuffle pictures" option, sooner or later you will notice that images from folders C and D are not displayed; may be because folder B has too many subfolders, or too many images, I'm not shure about the cause of this bug.
Luckily, I already knew about this new feature included in Vista called Search Folders, which are basically saved searches saved as files in a special folder called Searches in your user folder. Those saved searches are treated by Vista as virtual folders containing as files the result of the corresponding search. So, for example, if while browsing folder A, I search for "*.*" and save the search as say "recursive-slideshow", then I can set up the Image Slideshow gadget with the path to the opened saved search; I tick "shuffle images" and voilá.