Some programs you may find interesting

XCode & other tools by Apple, Inc.

XCode is the free coded environment that Apple provides for Mac OS X 10.3 and up. You will need to install the XCode tools if you want to process Mac OS X apps (the required nibtool utility is provided by the XCode Tools installation).

Resorcerer by Mathemaethetics Inc.

Resorcerer is the best resource editor for Mac OS. The swiss-army knife of the Mac developer, it starts where ResEdit stops. Keep in mind that if you want to write your own templates for PowerGlot to recognize your custom data resources, you may need Resorcerer.