Starcraft 2 Zerg training series: Annoy your enemy!

Zerglings are cheap. Zerg players tend to have a lot of minerals. Build packs of zerglings and patrol the map, taking out anything vulnerable.

Rush down undefended enemy expansions and break through the main bases defenses if possible, take out harvesters. If you happen to catch your opponent not paying attention, a pack of Zerglings can slip into the main base and cause serious havoc before your enemy even knows what has happened. You can cause a lot more damage than the cost in minerals.

If you are being seiged, odds are they will be steadily training new units and rallying to their main forces. Use fast units like Zerglings and Mutalisks to intercept these reinforcements as they stream across the map. An army stranded in Zerg territory with no reinforcements will eventually dissipate.