Floating Info Window
While an object (file, directory, disk image, etc.) is selected (most often in the Finder) and you hit Command-I you get an information window for that file. However, here’s a really cool modification: if you hit Command-Option-I the info window floats on your screen and isn’t fixed to the chosen object. Now every time you select another object the floating info window will change to show info about the new object.
This is really useful for moving through many files, directories, etc. because you only have to open one info window and it will keep changing as you click on the objects.