Two games for you today. Both are ZvZ match-ups with valuable lessons to learn. I used to have a lot of trouble with ZvZ until I learned how to progress past Roach-fests. The answer, as I've discovered it, is committing to air. Pulling this off without alerting the opponent is key. When you show up to the next fight, or his base, with 6+ mutalisks, you've probably gained the momentum -- exactly what a Zerg player needs.
Control the pace of the game so that you can tech to tier 3 while he's scrambling to assemble hydras. While getting Hive and Greater Spire, pump out 2-4 Corrupters and harass Overlords. When you can, upgrade them to Brood Lords and mop up!
This next game is a little different. My opponent was first to air, but I was anticipating it and had hydras ready to go. He completely backs off from going air and lets me take over the skies with Corrupters.