Windows only: Batch renaming utility Ant Renamer 2 has a simple interface that quickly automates the tedious task of renaming multiple files, saving you from aggravating your carpal tunnel.
To use, simply drag your files into the Files pane, and then select from one of the many available functions in the Actions pane. There are plenty of functions to choose, from changing extensions and string replacement to more advanced tasks like regular expressions or even using the XIF info from your camera. You can also chain multiple renaming tasks together into a batch, and preview the results back in the Files pane (make sure to use the F5 key to refresh the window after making changes). Once perfected, batches can then be saved for re-use—a great feature for working with downloaded files.
There are many other Windows renaming utilities like Bulk Rename Utility or Wildrename and Ken Rename and even a built-in trick to rename files sequentially, but Ant Renamer is an especially lightweight utility that offers a portable edition as a handy addition to your flash drive toolkit.
Ant Renamer 2 is a free download for Windows only. Mac users can check out previously mentioned Name Mangler.