In addition to the standard aerodynamic drag thingy depending on the original aerofoil pressure distibrution pattern and loading - think along the lines of sometimes increased directional stability eg Learjet or possibly increased lateral stability eg Lancair 360 homebuild and possibility of increased roll rate due to aileron flow pattern eg some sailplanes. The NASA website used to have a quick blurb on their early studies. I am sure the more enlightened out there can help think up some more groovy stuff.....
However, I believe (with no proof of any kind whatsoever) most importantantly a winglet is space for corporate advertising. Shareholder profits shall be held above all else. Long live the shareprice.
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By the way rhinodriver - did you get the job??