As an example of what I am asking I provided one answer:
Lettuce/spinach grow well in the empty spaces around the base of pepper plants. Pepper plants don't grow too tall and aren't particularly dense so light does pass though. In addition, the pepper roots like to go deep while lettuce/spinach typically do not (until they start to flower anyways). The height of the pepper plant also serves to shield the lettuce from heat, which is often desirable (to avoid early flowering).
Are there other combinations that make the most use of available soil and act as good 'partners'?
Are there other dynamics at play that might be useful in making the most of a raised bed?