I suppose all of us use some piece of software that's a bit off the beaten track, and it might be useful to name the software with an explanation of why it's useful.
I'd like to start this thread with my choice: Amok's ExifSorter. Over time I find that a single image gives rise to several edits, leading to file size changes and name changes etc. The one thing that remains constant is the exif data, and a search on the create date will list all the files with that date (year, month, day, hour, min and second), renaming them with this date information and appending copy 1, copy 2 etc for all the files it finds. The sorting process can be chosen so that it's possible to sort on several exif values at once. This can be invaluable for cleaning up what sometimes is a real mess. The interface is decent and might take a bit of getting used to, but I think it's worth it.
Do you have an oddball piece of software we might benefit from?