Two for the space of one!

Last year I grew many of my  vegetables in large (30l) pots and got into the habit of sowing more than one type of seed in each pot to make the most use of the compost, space and effort of watering & weeding. 

My unwritten rule was to grow a root with a leaf and a fruit. Not because of my Roots/shoots/fruits and Leaves plan but because they compliment each other with their space requirements. For example, a carrot needs room to grow below ground, but its not very demanding above ground so sowing lettuce or chard with the carrots  takes advantage of the same space.
All the tomatoes shared their pots with carrots or radish, and a leafy crop like lettuce or rocket. The mangetout and cucumbers that were growing on supporting canes were also undersown with roots and leaves. 

I know this is all very obvious but I wondered whether those of you who grow in open ground grow "space complimentary" vegetables in this way. Maybe lettuce at the base of your beansticks or dwarf peas and radish amongst the courgettes.
Veg that take a long time to mature, like parsnips and brussels sprouts have always seemed to me to be "greedy" plants that take up too much space/time and I've stopped trying to grow them. However, if I can grow other veg in the same space it wouldn't be quite so wasteful. 

I shall be experimenting with this soon, but, meanwhile, any suggestion for workable combinations that you have tried?