By "offensive," I mean that these plasmids are more suited to direct attacks.
Incinerate - With a quick motion of your fingers, you can shoot a massive fireball that will engulf enemies and set them ablaze.
Telekinesis - Any self-respecting science fiction nerd should know this one. Telekinesis allows you to move objects with your mind; useful for flinging objects at enemies.
Electrobolt - With this plasmid, you can fire bolts of electricity at your foes, momentarily stunning them.
Insect Swarm - One of the more fun - albeit disturbing - plasmids. Turn your forearm into an insect hive and send a wave of hornets at your foes. They'll be distracted for a while as they swat away their attackers.
Winter Blast - One of the coolest powers (literally), Winter Blast allows you to freeze an enemy in a solid block of ice. Afterwards, one whack of your wrench will shatter them into a million tiny pieces.
Cyclone Trap - Get ready to manipulate the weather ala Storm from the X-Men. With this plasmid, you can create a giant spinning vortex that will scoop up enemies and send them flying upwards.
Sonic Boom - This plasmid was added later in a content patch. Sonic Boom sends a shockwave that damages everything stands in front of you.