When I play against another Zerg, I'll send an early scout at 9 (just before overlord). If he does 6 pool, you can spot this earlier (low drone count and spawning pool 1/2 way done) and immediately chuck down your spawning pool (usually @ 11/12-ish). Get 1 spine up right in your mineral line and a queen ASAP. You should be able to get 2-4 lings, just as his first 6 arrive. Then attack move all your drones along with your lings and you'll fend off or kill his lings easily. Your queen and spine crawler should spawn during or after the fight but you'll have enough to defend any further attacks.
Now this is where it get interesting; If he only sent the initial 6 lings in, he's playing a psychological game which cause most players to overreact and build tons of lings or ground units to defend (instead of pumping drones). Don't try counter immediately. Keep in mind he'll be far behind in economy. Send a ling or two to his base and check if he's getting a lair if he doesn't have one yet, check for early non-natural expansion. Now you can either get banelings for a fast counter and try destroy his economy even further or mass macro. More often than not, they'll try tech to mutas, so get some early defenses or hydras or mutas if you are not planning to counter.
It's also possible for them to tech to roaches, but players that do this would be even further behind in the game as they won't have enough larvae for the roaches and overlords needed for the the high food cost - these players tend to get burrow and tunneling claws, so be weary of that. Since you'll have an advantage, you can either out-tech to mutas or MASS produce lings (sometimes getting a 2nd in-base hatch can work to your favour - this depends on the map though) and simply overpower his forces early on.
What I like doing if I'm going for that early counter, is split 6 lings in a separate control group, he'll probably have some resistance near his ramp and if you chase those back into his base, have those 6 run near the back (out of his vision) and morph them to banelings. Then rally those right into his mineral line, while keeping his other lings occupied at the front. Usually he'll notice them too late and you'll kill off quite a few drones. It's even funnier when he sends his forces to the back to fend off the banelings!
Most important; always check for expansions (overlords are great for this), because I've lost a few games where I thought he was cornered but somehow he could afford a LOT more stuff than I anticipated causing me to lose the games.
