I enjoyed Goodies' idea with the marbles, but I don't have any rep yet so I can't comment. I came up with a hard-sci-fi example just because I'm that bored tonight. If you wanted to do it right, and if you could accelerate marbles that fast, such a concentrated attack wouldn't need nearly the energy a meteor does. Let's talk about a marble traveling at a speed very close to the speed of light (like 0.99999c, around 10^12 MJ). It would really only take one. At such speeds, collisions occur on a particle-by-particle basis and they need to be handled from a nuclear physics perspective (hey, that's why I do. Let's have some fun!). In short, in the center-of-gravity system, the incoming nuclei are approximately infinitely massive compared to the air molecules, so considering deflection and slowdown, it's pretty much like the air is just not there. It's like shooting bb guns loaded with ants at an aircraft carrier. Sure, you may scratch the paint, but you're not going to change the direction of that ship (marble) even with a billion ant guns (air molecules). The point is, it will spread out a bit, but ALL of that mass is going to impact the planet's surface pretty much at the exact point you were aiming. Also, it doesn't really matter what the marble was originally made out of, since those nuclear interactions are going to quickly convert it into fundamental particles anyway. Marbles that started off as tungsten, steel, wood, chalk, or whatever, are all going to be approximately the same blob of plasma by the time they reach the surface. The only things that matter are the mass of the marble and its total energy (velocity).
Now here's the neat part. After it strikes the planet's surface, those collisions also occur on a particle level. The huge boom, splash, fireball, etc. like from a conventional projectile (asteroid) are all caused by friction and chemical reactions, so they just don't happen. To continue the sci-fi part, if our original marble was fast enough, there isn't enough planet to stop the infinitely-massive plasma-marble which just hit it. Instead, it simply starts burrowing a marble-sized hole which continues for a couple thousand miles. Deep enough to reach the planet's molten outer core. On its way through the planet, our marble transferred enough energy through those nuclear collisions to cause some really neat stuff. Imagine setting off a nuclear bomb halfway through the earth, the width of a dinner plate but 1,500 miles long. We can't produce sustained nuclear chain reactions with our marble, but we have created enough micro-chain-reactions to multiply the initial deposited energy a thousandfold. The resultant radioactive heating has melted everything within, say roughly thirty feet based on the typical size of the cavities from underground nuclear testing. Plus, we have a superheated plasma crust a couple millimeters thick that will heat things up even more. Boiling metal magma, anyone? All of that--let's call it superlava just for fun—is immediately going to start squirting out of the hole on the surface. After a few seconds, hydraulic pressure from deep inside the earth will have increased the velocity of the superlava jet to perhaps a few thousand miles per hour. THEN you get the fire storm noone has dreamed of before. Not the relatively cold mushroom cloud from a nuke, mind you. Pure liquid fire, hundreds of miles high, shaped something like one of those spinning tops, but inverted. Receiving an endless supply of energy and magma from the earth's core, this megavolcano--no, gigavolcano—will continue erupting for weeks, at which point the resulting ash cloud has completely enveloped the planet. Mass leakage and raining superlava has caused the oceans to boil for hundreds of miles around the impact point, which does eventually result in the mile-high tsunamis we all love so much—just a few days too late for Hollywood's tastes. So yeah, say bye-bye to every living thing on the planet.
Oh, and the backup plan—that space station in orbit full of survivors? Kiss all your LEO objects goodbye. If the EM pulse from the marble's initial impact didn't take it out, their orbital path will eventually take them right through that superlava geyser. Imagine frantically trying to survive in space for days without power, only to get melted. Bummer, man.
Yeah. That was fun. It's obviously quite fictional, but I did perform a reasonable estimate of the energy required to poke a hole into the earth's core, and calculated the energy needed to raise everything in a 10-m-wide column to boiling temps (appx 3000K). So at least that part of the physics is consistent within a factor of a couple :).