Free software Rename-It! integrates with Windows Explorer and can rename multiple files quickly and easily.
Select multiple files in a folder or several folders (like photos you just downloaded from your digital camera, say.) Right click in Windows Explorer, choose Rename-It!, and define the new file name pattern. Search and replace words in filenames, convert to upper or lower case, or crop filenames to a certain length. Rename-It! also supports ID3 tags (music file information) and regular expressions. Free, Windows only.
Rename-It! [SourceForge]
Mac users, see Rename files in bulk, the Mac edition