Is there any keyboard hotkey to undo hiding an application? I use Command+H to hide applications all the time. Sometimes I really want to undo that action, but pressing Command+H again will just hide the now active application.

Pressing Command+Shift+H doesn't do it either.

I don't know if there's a way to do this built into OSX, but if not I wouldn't be opposed to installing an application that would provide this functionality.
Doing Command+Tab will let you go through all the running applications, even if it's hidden. More tricks are discussed at SuperUser.