Irfanview works well
For a standalone,
14Rename -
http://www.snapfiles.com/get/14rename.html - is the best renamer I've found.
"1-4a Rename (One for All) is a powerful mass renaming tool, that can modify names and extension for a large number of files at once. It offers various renaming schemes, many of them not often found in other tools, as well as preview of the new file names and complete Undo. The program allows you to convert case, add prefixes, add incremental numbers, remove spaces, replace characters and more. Standalone, no install needed."
Oh, and freeware, too...