Ive found that using Goo Gone is the best solution IMO. they sell it at Lowes or Wal Mart and im sure other places. What i always do is get a Q tip and spray the Goo Gone on the Q tip. Rub the marker a lot and it will come off. Just dont scrub to hard as it might tear up a label. Dont worry about it messing up the plastic though, ive never had it mess up the plastic at all. As for marker and sticker residue on the labels of NES Games or SNES Games. Make sure to use the Goo Gone lightly. It dont really seep into the label unless you get it around the edges. Then it seeps under the label and looks like a wet spot. So be careful when using it around edges and dont use too much. Its more of a greasy kinda solution. Ive also used it to remove sharpie marker from the top of CDs as well. Hope this helps :-)
-Mark