Above: a large patch of clover in my lawn stays greener than the grass all summer and doesn't mind being stepped on.
I have written about the benefit of clover in lawns before - I have purposely dug up clover from my garden and the occasional back alley source and brought it home to plant in my small lawn. Clover is a nitrogen fixer which means it takes nitrogen from the air and puts it in the soil where it can be used by plants i.e. grass. Plus, I don't mind the look and the flowers even smell nice! It can take over the whole lawn for all I care.
My friend and I were just chatting about this the other day and I told her I had read in several places (but hadn't seen any hard evidence) that clover used to be a common plant in lawns until the pesticide industry convinced us that it was a weed. Now I have an actual reference thanks to this excellent post from May Dreams Gardens!