What's the chemical nature of a dishwasher's rinsing agent? It seems that a rinsing agent helps the dishes in the dishwasher dry much better than they do without it (which I recently noted on my new dishwasher). What molecules are used in the composition of these rinsing agents? How do they help the plates dry?
According to the MSDS for Jet Dry (a common brand name) it contains:


Sodium 2-phenylpropane-2-sulfonate (Sodium cumene sulfonate) 
Propane-1,2-diol 
Citric acid


I think it may be safe to assume that the citric acid is in there for a "clean" smell and also to soften the water (thanks to the commenter).  The sulfonate is presumably a surfactant to help break the surface tension of the water, and promote evaporation, and the propylene glycol may help serve as a coolant (and is favored due to low toxicity).