Vs. Terran: Banelings and Mutalisks. Send in the Mutas first to drop any siege tanks defending the enemy base's entrance, then roll in and melt all their units and missile turrets with Banes. then clean up the buildings with your Mutalisks. If the first wave of Bane/Muta doesn't get the job done, quickly make another wave and try again.
Vs. Protoss: This is the only race that gives me a lot of trouble. Best advice I can give is to use Roaches and Hydralisks. Send a wave of ten or twelve units ASAP and keep the pressure on the enemy base with larger and larger waves. Don't let them horde up on stalkers and zealots or you're dead.
Vs. Zerg: If you're in the bronze league, chances are they'll 6pool zergling rush you. Just attack move (hotkey A) past the zerglings with all of your drones, and you're guaranteed to win. Then build up defenses, tech up to pretty much anything, and win. If you're facing a REAL zerg player, tech to fully upgraded Hydralisks ASAP. Then follow the usual growing waves strategy.
Hope that helps some of my fellow Zerg users. If you're a better player than me, feel free to critique and correct in the comments.