Rename It

Rename It is a free
Explorer shell extension that can easily change the extensions of one or
more files. It can also rename a series of files in one operation, such as
image01.jpg, image02.jpg, image03.jpg, etc. Just right-click on highlighted
files in an Explorer window and choose one of the Rename It menu options. A
stand-alone executable is also included for renaming groups of files from
other applications that don't use the Windows context menus but have their
own Explorer-type interface (such as ACDSee). Every rename operation can be
undone from within Explorer.