Rename-a-Batch is a very simple file renaming tool. It can be used to rename one or multiple files very quickly. The application couldn't really be any simpler. In fact, the user interface consists of a single window, where you can select the files that you want to rename, and then tell the application how you want to rename them.
The app only lets you rename files by providing a new file name. If you want to change the number from which files are numbered, you can also do that, and you can even tell the app to add a zero before the numbers. So, if you have a couple of hundred of images with the generic file name IMG_XXXX, you can use this application to rename them to the name of the event or place where they were taken, for example.
There are other file renaming utilities that let you customize the process further. But Rename-a-Batch is a very useful app that keeps things simple by just letting you do one thing. At $0.99, the app is inexpensive and it works really well.
In my testing, I was able to rename a couple hundred pictures in less than 2 seconds.