For those of you who didn't see the IEM finals they are worth watching.
I've moved the recap down below my explanation
When faced with an early Reaper harass, usually Speedlings plus the occasional Spine Crawler and Queen are sufficient. However, mass Reapers are a very different story.
Mass Reapers usually have speed upgrade putting them at Speedling speed off creep. As a result you need to either get a good surround or engage on creep to win. The natural response to Reapers is Roaches, but without a Speed Upgrade or creep, Reapers are just too fast. Once you have Roach Speed (and can use Overlords to spread creep as Reapers only attack ground) you can usually shut down this mass Reaper play.
Edit: MarroW also suggests throwing down extra Queens, which I think is sound advice. Though the Queens won't help with pursuit, they will help with extra Creep, Transfuse, and longer range if he makes a solid Reaper push. Likewise a Creep Crawler couldn't hurt, but won't help much as once he has 12 Reapers he can almost 1 shot it.
What made it so effect vs Idra was the follow up with Marauders. Based on this its important to prepare for the follow up:
Since the ideal build for Marine/Marauder is Banelings followed by Speedlings (or Hydra), as long as you get your Baneling nest out and upgrade speed, you should be in good position to deal with this switch and push on to victory.
There is a good break down of game 3 by Day9 if you want to skip ahead to it.
As for the IEM finals, allow me to give you some context. Marrow (who originally wasn't to be part of the IEM championship but was brought in after BradOK was denied a Visa), plays a very aggressive 5 tech lab vs Idra. Idra is well known as a very passive, very good, zerg player who prefers to fight late game battles with a superior macro style.
In this context, early harassment is a very good strategy against such a passive player as it denies him his economic advantage. In earlier rounds we saw Sarans use harassment effectively vs Idra and pushed him to Game 5 (its a best of 5). So its not unlikely that Marrow studied those matches when he decided on this build.
I want to lay the ground work here so you can see that this was very much designed with Idra in mind.
The build itself revolves around 5 techlabs to push mass Reapers and force Idra to build a lot of Roaches. Ultimately Roaches with speed (or on creep) beat this early Reaper play, but the goal here isn't to win with Reapers, but rather to force Roach production and follow up with a heavy Marauder force (which also require Tech Lab, so your infrastructure is in place).
In game 1, Idra adapts to this change and pushes with a heavy Baneling/Roach/Speedling force, and almost wins. Its worth noting that as Marrow's eventual strategy is MMM, forcing a large number of Roaches gives a very tiny window for victory. What's more, if Idra had Baneling speed upgrade at the time (which he was researching) he would have easily taken out the remaining Marines and pushed on to win Game 1.
In game 2, we see the same strategy again, but with very different results. Idra gets an early Roach speed to shut down Reaper play, and then follows up with Mutalisks to shut down the Marauder push. Marrow makes a good last ditch effort with Ghosts, but ultimately doesn't have the force. Idra decides this fight with Banelings (speed upgraded now).
In Game 3, we see a much more confident Idra who counters the early Reapers with just Roach/Speedling. He'll go on to lose this game much later after a disastrous assault at 12:30, and ultimately get steam rolled by BioMech before he can get more than 2 Ultralisks up.
As Idra strongly believes BioMech is imbalanced vs Zerg, its not hard to imagine a very defeated Idra in Game 4, not playing up to his potential.
Here is the Morrow's recap of the build order and (in his second post below) his answer to the question of best counter.
The best effort in those finals was the second match, which was basically a result of spreading creep tumors, using zerglings/queens to survive, getting lair tech early, using overlords to generate creep everywhere, and getting mutalisks asap to return the favor. Not sure how well this would work for the average player, because controlling a mass of reapers is pretty straightforward; but it seemed to work alright for IdrA. If your opponent is only investing in reapers and marauders, then using air units against them will work wonders.
Try it and let us know? I imagine every man and his dog is doing 5 rax reapers online at the moment :(