it's not an add question -
i think skrommel is so used to using "AutoHotkey" that he forgets that most of the world has never heard of it.
all of skrommel's utilities are written in a very cool free scripting language system called AutoHotkey:
http://www.autohotkey.com/it's definitely worth installing since you'll probably be seeing more scripts from skrommel you like

most of the time skrommel adds tools to his page but sometimes if its short like this one he just pastes it in a reply.
so you would copy and paste his code and save it as a file with a .ahk extension, and then after you install AutoHotkey you can run it (or compile it into an exe using autohotkey tool).
while i'm here i have a question for skrommel:
1) can we drag and drop files onto this autohotkey script from explorer to have them renamed?
2) if we compile the script can we do this?
3) in your line setting the separator, would there be a way to specify a SPACE character as a separator?