This free app does what it says it does, quickly and flawlessly. I've been using it for a few years and though it hasn't been updated in a while, it still works great, even under OS X 10.7.
I use it primarily to rename folders full of photos to reflect the composition, such as "2010_08 Chicago Tom xxxx" where Tom is the photographer and xxxx is the sequence number.
It's easy to use: Drag a folder to the "Choose Folder" field, open the "Options" sidebar, insert a string of characters in the "Prefix" and set up the sequence setting, and you're ready to go. There is a "Test Change: button that will change the names of the files in that folder in test mode so you can see what they'll look like. If they are satisfactory, then clicking the "Do Change" button will effect the desired changes.
A gave this 5 stars for Features as its feature set is as good as it gets: simple and to the point.
Five stars for Ease of use because it is extremely simple to use - no Help documentation is needed (none is provided).
Five stars for Value as it just works and is free.
Five stars for Stability as it has never crashed or misbehaved in any way as I used it from Leopard through Snow Leo and in Lion. This is the way an application ought to work.
While there are other apps out there that have more features, they get more complex and trying to set them up gets tiresome. As usual, your mileage may vary as those apps might be just what you need.