Marlin 1894 .375 magnum. Not really 'out there' but it's a pistol caliber weapon, and they banned pistols so I suppose I'm a bit of a rebel.
Remington M1100 practical shotgun with 9 round magazine. Great for clays and PSG, probably the closest thing to an 'evil assault weapon' you can get here (well, I suppose you could get an USAS with a drum mag or something).
Straight-pull AR15. An AR15 that is operated by pulling the charging handle manually. Again, not a whole lot different to a regular bolt-action rifle, but it looks evil I suppose.
Ruger Old Army. It's a pistol, but since it's muzzle loading, it's legal.
I don't have one, but plenty of people have Ruger .375 long barreled revolvers. Basically the barrel is just long enough so that it's not classes as a handgun in law and is therefore not illegal.
There's also some German rifle, I forget the name. Pump action rifles (not shotguns) are illegal, but because of the definition in law, this one is legal due to the fact that you don't pump the fore-end, you pump the pistol grip itself. It's also bull pup, so is extra-evil.
Plenty of .50 caliber and .338 lapua shooters with all manner of very large guns.
Probably the most ingenius invention to get around the law I've seen is still in development. It's an MP5 clone that ejects the empty case like an ordinary one, but locks the bolt in the rearward position. You then just flick a switch, positioined a little like a selector switch, to let the bolt move forward and load the next cartridge. Because it requires an input by the user to load the next round, it is not self-loading, and thus legal, but wouldn't have a rate of fire much less than a semi-auto MP5.