Source: Codejunkies

This cool device slots into your GBA or the GBA slot of your DS and enables you to access cheats and manage game saves. It comes with a PC application which you can use to download cheats and saves from the CodeJunkies server to be transferred to your DS or GBA.

Isn't it annoying how GBA carts usually only have room for one game save? Well, with Action Replay Duo you can back up your GBA and DS saves to the Action Replay vault itself, or on your PC. Great if you share your GBA or DS with your friends.