The HUGE finds
If you ever see any of these games at a boot sale or second hand store, don't let them see your excitement. Tell them the sweat trickling down your cheek is a medical condition and those palpitations shaking your body are hereditary.

Air Raid – Atari 2600
It's tacky to look at, isn't it? No title on the game cartridge, a weird handle to yank it from the system and a sky blue plastic finish. Not to mention poor artwork. But don't let its appearance put you off – this is an incredibly rare game. There are FIVE known copies in existence and none of them have a box or instructions with them. So if you find number six… well, expect it to sell for at least £1,500 ($3,000). A boxed copy? Who knows. You could name your price.

Stadium Events – NES
This was an official third-party title for NES in the US, using an early precursor to the Wii Fitness board – the 'Family Fun Fitness Pad'. Nintendo liked it so much, they asked for all of the NES versions to be recalled, made it wireless and rebranded it as Wii Fit.

Nah, just kidding (sorry if we had you going). No, seriously, the 2,000 copies produced were recalled, leaving a couple of hundred games in the hands of customers. The game was rebranded as World Class Track Meet and distributed as a first-party game, with Nintendo's own 'Power Pad' controller. But those scant few copies of the original still in existence now fetch over $1,000 (£500) when they appear – and complete versions are rumoured to be worth around $5,000 (£2,500).

Ultima: Escape from Mount Drash – Commodore Vic-20
A Commodore Vic-20 was this writer's introduction to the gaming world. Ah, happy memories of Frogger, Blitz and Initial Race. And taking it literally when it said 'HIT SPACE BAR TO CONTINUE'. Thwack! But anyway, this Ultima game is hardly anything to do with the main Ultima series and Richard Garriot (Lord British) didn't even know it was being produced with the name until it was out in the shops. It sold badly even before it was discontinued, making the remaining few copies that did make it to consumers' hands worth something in the region of $3,500 (£1,750).

Kizuna Encounter – Neo Geo (PAL)
This is quite unbelievable. Apparently, there were 12 (yes, twelve) copies of this SNK fighting game produced for the PAL territories. It is said that eight of those were shipped back to Japan and relabelled for sale there. So that makes… four copies. They're doing the rounds in Europe and the US, selling for about $12,000 each time. Yes, that's about £6,000.