#FreeSoftwareAdvent day 21: Thunar
Thunar is the XFCE file manager. I don't use XFCE, although I have used it in the past on old machines. I don't use Thunar as a general file manager, but I do use the bulk rename functionality, which I find very useful.
I've tried other bulk rename programs, but I prefer Thunar. The interface is fairly straightforward, with a list of files to be renamed in the left column, and a preview of what they will be renamed to in the right column.
Below that is the renaming functionality. There are various options, including inserting/overwriting, numbering, search/replace (with optional regular expressions), etc.
Whenever I need to rename multiple files, I find Thunar to be a quick and easy way to do it.