I've tried the beans/corn technique that Sam Salmon mentions. Notice in the picture there is also squash.
The idea is that the corn stalk makes something for the beans to climb, and the squash, with those prickly leaves keeps unwanted critters away. I think this last part is mostly wishful thinking, since anything that wants to get at that corn isn't going to be slowed up much by some squash leaves.
Anyway, it's tricky to actually pull this off. Your timing has to be just right. If the beans get ahead of the corn, then the beans just smother the corn, and if the corn gets much ahead of the beans it shades the beans--in any case, you can only really get away with planting the beans on the outside and southfacing row; otherwise, at worst, one or the other ends up overcoming and drowning out the other one. At best you have a tangled mass that's hard to penetrate for cultivating or weeding or for picking either the corn or string beans.
Squash, by the way, is another heavy feeder.
If you grow winter squash, here is something you might be missing out on: those squash will keep setting fruit in the fall way past when they have any chance of maturing. When your squash get to that stage, start picking the immature squash while they are really small. Think of them as zuccinis, but better. Also try this: pick a bunch of the male blossoms early in the morning and add them at the last moment (say five minutes before it is done) to your favorite chowder base. Tastey, and the pollen makes it a very nice color