Batch File Renamer could've been called Easy File Renamer, because although it's cursed with a bland layout, that belies the effortless renaming of massive quantities of files.
The features combined with the tiny size make this one of the best freeware file renamers around. You can set entire directories to rename, and select all files, or filter out the ones you don't want to change by their extension. A handful of presets let you convert filenames to upper or lowercase instantly, as well as replace characters in the filenames, add a number sequence to the name, and for advanced users you can even run your own command line on source directory files.
The program runs from a self-contained EXE that takes up a whopping 48KB on your hard drive. Double-click to get going, and you'll find yourself presented with an uninspired but still utterly utilitarian layout. In fact, the app is a hummingbird of a program, so light that you can't even minimize it--watch out that it doesn't get lost under a pile of open windows.
The top half of the program is for selecting the source directory and the file type. Besides selecting all, you can choose DOC, TXT, OCX, JPG, HTM, and more. Directory navigation is accessible from the menubar, but it feels archaic and it's probably easier for users who want to change files in directories 10 levels down to copy and paste.
The bottom half of the program is where users can choose what changes they want enacted on their filenames. Presets include case changing, changing the specific filenames to generic naming conventions followed by numbers, and character replacement.
You can also change extensions en masse, although it's not straightforward. Choose one of the filename change presets that doesn't do anything drastic--one of the case changes, perhaps. Then select the extension box next to the presets, and type in your preferred extension. Hit the Preview button and you should see your new file extensions in the window, then hit Start Renaming to complete the changes.
Although there's no doubt that a modernized interface would be nice, as would a Help file, Batch File Renamer is light on its feet and probably does exactly what you need it to. Do you have a favorite file renamer? Tell us about it in the comments below.