The B777-300 and -300ER has 3 external cameras fitted (as standard I believe). One in the leading edge of each side of the tailplane facing the main gear and one belly-mounted, facing the nosewheel. They are intended to assist while taxiing/manoeuvring.
The output from the cameras is displayed on the selected EICAS screen in a split-screen format. Needless-to-say as soon as you have become accustomed to the long wheelbase of the -300 variant, the cameras become of less importance. They are particularly useful on the gate to monitor the loading/refueling progress.
Cheers. ST.