Mac: Yosemite has a bunch of cool hidden features and iDownloadBlog points to another nice little feature: a new batch renamer.
All you need to do is select a group of files, right-click, and select the Rename option. You can then use a Find and Replace style renaming, add text, and change the entire name and format. It's a super-easy way to batch rename files and it's nice that it's finally integrated into OS X.
How to batch rename files in OS X Yosemite [iDownloadBlog]