Sega Mega Key?

You'll need the Mega key with dip switches on the back to get true 100% region functionality.

I can confirm that they work with absolutely everything though. I've yet to find a game that it cant get around the lockout on.

One problem though, they're really hard to find! One on ebay at the moment but I would expect for it to end about €50.

http://www.ebay.co.uk/itm/SEGA-MEGADRIVE-SUPER-MEGA-KEY-GAME-ADAPTER-BOXED-NEW-/300583610236?pt=UK_PC_Video_Games_Video_Games_JS&hash=item45fc2de77c

The down side to them though is the game'll be playing in 50hz. You're better off having your console modded.

There are two types of region lock.

Physical lock (the cart can't fit the console) - Game Genie can get around this. But don't bother, as all you have to do is remove the two little plastic notches from the Megadrive case. You don't need a Game Geanie at all for that.

Then there's the real lock outs. Which Game Geanies are rubbish for. Theoretically you can get codes to get around these locks, but there are more than likely no codes written for the games you want.

Then even with the codes,you'd have to manually enter them every time you wanted to play the game.