There is an endless list of apps that allow you to make video calls with your iPhone 4, and aside from Apple's FaceTime, most of t hese are also compatible with the 3GS, but it can be quite awkward to video call without a front facing camera.
Korean company, PLAYMASS, have produced a new accessory that can solve the problem, iSeeU.
iSeeU clips onto your i Phone 3GS, and flips open to reveal a series of mirrors, that allow you make a video call using the rear facing camera. One neat thing about iSeeU is, you can easily swivel it off to the side to switch the cameras, so that you can still show your friend what the rear facing camera would normally see.