Attracting beneficial insects
 

Nice little article outlining useful insects vs bad insects. It also tells what kind of plants attract the good ones.

Attracting Beneficial Insects to the Garden: Control Pests in the Vegetable Garden with Natural Predators | Suite101.com

Interesting that the article doesn't identify one of the more efficient predatory insects in the garden, yellowjackets and paper wasps. They are very efficient predators of caterpillars as they work through the foliage looking for cabbage loopers and the like, especially later in the summer as their colony size increases. Their nests aren't welcome in my garden, but they are welcome to visit and forage all that they want to in my garden. Another natural predator are tree frogs and toads, both of which frequent my garden.

As far as attractant plants, you don't need to add plants especially for this, just let some of your carrots, fennel, parsnips, dill, lettuce, and corn salad go to flower and they will attract plenty of insects. Members of the Apiaceae are especially good at attracting pollenators