What I do depends on the circumstances. If family or friends invied for a meal, I will always take flowers or wine or both with me.
With children, it is more complicated because one lives 100 miles away and the other 200, they do not visit just for a meal, they come and stay, anything from a weekend to a week and we stay with them in a similar fashion. The exchange will usually involve flowers, then, if we have recently been to France, Brittany cider for DS, little bottles of red wine for DDiL, bits and pieces for the children. Also anything we have picked up which we think they will enjoy/find useful
DD arrived on Thursday evening bearing a Christmas present, a sweater she was knitting that took longer than expected to complete, a heap of magazines, to match mine. It is an Easter tradition, as we usually go to France for Easter and buy the magazines to read in the (tv-less) evenings, 2 wicker storage baskets she no longer needed, a jar of homemade pickle and a couple of DVDs.