Download The Gimp and you'll know what this is all about.

• Photoshop-ish Keyboard Shortcuts for The Gimp


• Rounded Selection plugin
This Script-Fu plug-in rounds the corners of a selection. If no selection exists, a selection is made of the entire layer size and the corners are rounded.
   

Download and copy the Rounded Selection script to the scripts directory.
Linux/Mac:     ~/.gimp-2.2/scripts
Windows:     C:\Documents and Settings\<user name>\.gimp-2.2/scripts

Upon restarting the Gimp, you will have a new menu option named Rounded Selection under the Select menu entry as referenced in the screenshot below.