What winglet airfoils are used on the Boeing B737?
Aviation Partners Boeing, a joint venture between the two companies, developed the winglets for 737NG, 757 and 767 aircraft.  They are offered in two varieties, a "blended" version, which is where the wingtip is upturned to near vertical and a newer, "split scimitar" version which adds a downward pointing tip as well.  They are made of a composite material with aluminium leading and trailing edges.

Their purpose is not to generate lift, but rather, prevent wingtip vortices (which generate a significant amount of induced drag) from spilling onto the top of the wing.

The design characteristics of the blended winglet are described in US Patent 5,348,253.