One of the best gardening things we have learned (from experience) is how valuable lettuce can be!
When lettuce is planted in a group (such as 12x18) instead of the usual rows, the leaves will spread out from plant to plant. This will block out the sun (thus killing) any weeds trying to grow between the plants.
We also use our lettuce transplants under our broccoli and cauliflower plants. Not only does it give us a double crop: it acts as weed control for the broccoli and cauliflower similarly like it does in the group planting example above.
By planting our groups of lettuce every couple of weeks, we have fresh lettuce right up to the first frost.