What do you mean by a file manager? Command line or GUI based? Do you need it to be full featured, or are you just trying to do a file selection window, or something like that?
If you'd like a GUI based one - check your OS. There's probably hooks for manipulating the OS's GUI file manager. They may not be offered natively in Ruby, but more than likely they're available in C. You can wrap the hooks in a ruby extension, and then do what you want.
If you'd like a command line like one - check out FileUtils. It gives you the hooks you need to find out the directory contents, move, copy, or rename files, and pretty much anything else you want to do.