I just had a match where a few well timed banelings saved the game for me.  It was  a Zerg vs Zerg match where we were next to each other on the map.  I though I was off to a bad start.  My normal build is 13 pool, but with Zerg I tend to 10 pool.  I made a huge mistake and built an evo chamber instead of my pool.  I scouted his base and saw an early expansion so I also expanded quickly.

When the building that I thought was my spawning pool finished, I realized my mistake.  I put down my pool and got drones on gas.  I decided I was going to skip my queen and get a Lair.  I was able to get a quick 8 zerglings from my 2 hatches and rushed them over.  I had bad micro and ended up having to pull back after loosing a few.  I sent them all into the other base, but I didn’t get them grouped first.  Watching the replay, I had mine out slightly before his.

He attacked me with a few roaches and a good deal of zerglings.  Just as I was getting my hydras out.  My initial troops fell to the attack, but I was able to hold it with new troops.  He sent a bunch of zerglings as a second attack and I was able to hold that too.  I went to counter attack but pulled back when I saw 3 mutas fly over head to my base.  My hydras made short work of them.

I had full view of the map the whole game.  I made a 3rd expansion that he almost didn’t know about.  I had both watch towers on the map and I had a zergling set to patrol the rest of the map.  That patrol unit made him cancel one hatch that it discovered and killed 4 other stray drones that were going to expand.  Twice he got a drone close to my 3rd hatch but the patrol zergling picked him off.  I also had an overlord on a cliff next to the front of his natural expansion.  He would gather all his ground troops in my view and he had no clue he was there.

So I know he has mutas someplace but I see him build this huge ground army of zergling.  I am trying to build my hydra zergling to hold it off.  He keeps adding more and more zerglings.  Then I remember banelings.  I toss down the building and it finishes just as his army moves out.  My army was pooling at my natural and I pull it up the ramp when I saw his army move out.  Why he is working my my hatch, I am letting 5 banelings finish morphing.  His mutas attack my main from the worker line and I am there to clear them out.  When his zerglings run up the ramp, my 5 banelings take out 33 zergling.  I clean up the rest of them with little losses and counter attack.

I rush his natural.  My troops run into only a small group of units.  With them occupied, I send a few banelings into the mineral line and clear out most the workers.  I clear out all the units there, and head up the ramp.  I move into the base and he gives the game to me.  He had nothing to hold me with.

Well placed banelings will just melt mass zerglings.