This unassuming, unlicensed Nintendo Entertainment System game is actually worth a small fortune if you find it in a box. This cart is currently going for $200 on eBay, which is actually pretty reasonable. 

Caltron made a number of unlicensed products for the NES, but this one is by far the best. Keep in mind this is still a quagmire of intellectual property theft and bad design. The six games are all completely unrelated, but one resembles the Sega Genesis title Space Harrier,and another features completely stolen Gundam assets. 

I only know what's inside the Caltron 6-in-1 because I've actually played it, but regretably started running my mouth about how rare it was before I could buy it for a reasonable price. One of these carts just materialized in the game store I work at and laid buried in a drawer for nearly two years before I said anything.

Pro tip: Go ahead and buy rare things when you know no one around you knows what it is....