This awkwardly named utility (now known as "Better Rename" in the Mac App Store) can be extremely useful in some very specific situations. Anyone who needs to rename large batches of files on a daily basis--especially Web developers and anybody working with lots of photos or audio files--should definitely check out A Better Finder Rename. Most of what this app does can also be done with the Finder and Automator, but nowhere near as quickly nor as easily.
ABFR offers a huge number of renaming options, everything from modifying text and adding sequence numbers to changing formats and renaming files based on date and time (especially great for photographers). You also have numerous options for inserting text and file path components in a particular position. Plus you can specify, apply, and save multiple renaming steps, and ABFR can automatically resolve filename conflicts. You can do all this using ABFR as a standalone application with a new single-window interface, or you can access its functionality through the Finder's contextual menus. ABFR also offers a quick preview feature (which can save you from making sweeping "batch errors"), and it even supports regexes for more geeky users. The only drawback of ABFR is its price, which is relatively steep if this isn't "must-have" functionality for you. The current version is no longer compatible with Mac OS 10.5 and earlier.