What do they look like?
This time of year the huge black or brown and yellow cicada-killer wasps start hovering around and digging their nests in the dirt of my garden. When the den is finished some poor cicada will be captured and dragged to the hole and then the eggs will be laid with the living bug as a fresh source of food when they hatch.
They're big, and they'll buzz you, but I haven't found them to be particularly aggressive. On the bright side, it means you have a healthy yard and haven't killed them off with garden chemicals. You'll find plenty of discussion of them here in the forum.
This is the library entry about the cicada killers.