Tetris (Tengen Version) NES $15-$60

This is probably the most well known "rarity" on the system. Back before Nintendo released their Tetris...Tengen (who had been making unlicensed games on the NES) released their own version. Nintendo filed a lawsuit against Tengen, and they were forced to remove their Tetris from the store shelves...which was quickly replaced by Nintendo's version. This game used to be considered one of the most rare games on the system...but people have discovered over the years that it is a lot more common than they originally thought. Sure, you most likely will never find it in the wild...but there have been tons of copies popping up on Ebay. A loose cart will sell for around $15-$30, while a complete copy typically goes for $40-$60.