Bulk Rename/Merge Categories Wordpress Plugin
Upgraded version for the power user of Rename/Merge Categories plugin by Macedonian WP Team.
Use this plugin to rename and merge multiple existing categories. The difference is that it uses checkboxes and radio buttons instead of an inneficient dropdown list. Also, the “Delete first category” option is now checked by default.
Here’s a screenshot : 
Works with Wordpress 2.5+
Download Bulk Rename/Merge Categories Wordpress Plugin
Comments are welcome. Good luck!