Batch Rename Files Instantly with Name Mangler
Name Mangler is an indispensable app for OS X that allows you to batch rename files. I first bought it back in 2010 and have used it on a daily basis since. It's built by Many Tricks which is a well known indie Mac developer (of Butler fame).
Name Mangler takes a file or folder of files and renames them according to the logic you set. This includes things like changing case, find/replace, adding prefix/suffix and many more complex operations. All you have to do is drag the files or folder you want to rename into the drop area, set your rename logic and click rename. However, one of the coolest features are "Droplets".
Droplets are essentially rename presets you save as an app (they have .app extension). You can then place these Name Mangler Droplet apps in your dock or as a favorite in your Finder sidebar. Once a droplet is created you simply drag files on top of the app in order to rename the files. Name Mangler doesn't actually open - the process happens in the background.
The main use case of Droplets for me is changing a file's name/extension to lowercase and then replacing spaces with underscores (so FTP ready). I use the "Bracco Rename.app" in the screenshot below is a Droplet that performs the above rename actions.