NibUnlocker

NibUnlocker is a free and easy to use Mac OS X application that will attempt to parse a non-editable nib file and output a xib file, editable using Interface Builder or Xcode.

The resulting xib files do not contain all the information that was in the original xib file used to create the nib, however, so the xib file created by NibUnlocker will not be a complete replacement for the original. It can be very useful for examining a nib file and seeing how it is constructed.