Underscore is a command line program to replace spaces with underscores in all filenames in the directory it is called in. 

Directions:  Add Underscore to your PATH.  Navigate to the directory containing the files you want to rename and run Underscore

Notes: This program was tested with Ruby version 1.8. The .exe, however, does not need any version of ruby installed.


Underscore (.exe file)

Underscore source (Hosted on GitHub)