I had been looking for something that would allow me to add drop-down menus to a horizontal bar that listed my pages. Transmenu allowed me to add drop-down menus for the Subpages, which was just what I was after.
Many other dropdown menu plugins didn't quite seem to work. I'm still getting my head around using this one but it does work pretty well.
I added some code to my header.php and footer.php and you may need to adjust some other bits of code to ensure it reads all the other files properly (eg the correct directory) but if you're after a dropdown menu for your subpages, this is a good one to try out.