Rename Files

Rename Files enables you to modify the names of multiple files at the same time using numerical and alphabetical incrementation. It lets you specify the starting number or letter, as well as enter a base name to append the numbers or letters to.

Portable tool with a simple UI

This is a portable file renamer, so you can save it anywhere on the disk or a removable storage unit, and just double-click the .exe to launch the app. It doesn't modify Windows registry settings.
It's packed in a classical-looking interface that, although it doesn't put emphasis on graphical elements, it's simple to navigate.

Bulk file renamer with incrementation

You can drop multiple files into the main window, view their full path, enter the base name (this field can remain empty), as well as select the numerical or alphabetical naming mode.
Moreover, you can choose the digit format, specify the number or letter to start from, make the letters all uppercase or lowercase, as well as ask the tool to automatically sort the files by name on task completion. A dialog pops up to ask for confirmation when attempting to proceed with the task, but this can be disabled to speed things up.

Observations

There are many renaming tools that create new files with the new names instead of changing the original ones, in case you're not satisfied with the results. However, Rename Files changes the original items, so make sure to create backups if you want to avoid this scenario. Unfortunately, there's no option for previewing results or simulating the renaming job.
The file extensions are not affected. We've noticed that it's not possible to add new files to the task list with existing files, since the ones which are already there are replaced with the newer ones. This means that you must add all target files at once.

Conclusion

Rename Files is an old software application wrapped up in a rudimentary interface, and it comes with a couple of inconveniences. On the other hand, this file renamer carries out tests fast, and it works on recent Windows editions.