MacMan and the Caber Eater, a simple educational game for the ZX Spectrum hell-bent on teaching 80s youngsters how to subtract, is quite the Speccy rarity and easily one of the most expensive bits of software a collector can nowadays buy. Also, it sports a fantastic name and was published by Sinclair and Macmillan (!). And you can even try it for free courtesy of the ever-excellent World of Spectrum.
To actually own the thing, do give this MacMan and the Caber Eater (ZX Spectrum) eBay auction a look. It's the complete, finely preserved, boxed game and the seller is shipping worldwide.