This video shows how to remove a rental sticker from a Nintendo NES game cartridge! All you need is: 1) Goo-Gone or De-Solv-It 2) 1 cotton ball 3) A smidgen of patience
This is a great way to make mediocre quality games into top-notch collector's pieces. It is not recommended you use this technique if the rental sticker is on the label.
Goo-Gone is used in this video, but it might be caustic enough to damage the plastic. If you have a problem use non-caustic De-Solv-It. This will work with any old-school plastic game carts.