What is the source for what looks to be small trails of smoke or condensation sometimes coming from the very tips of a jet aircrafts wing tips?
These are wingtip vortices, which also appear in formula one, and this article explains the reason quite nicely (including airplane pictures and going into the difference between them and contrails).
The significant paragraph:
After the formation of the vortex, the pressure inside it decreases significantly as compared to the ambient pressure. This results in a significant drop in dew point (which was already below the ambient temperature before the formation of the vortex) but along with it, there is an even greater drop in the vertex core temperature, which transforms the water vapour into water droplets and hence we are able to see the vortices trail coming out of the rear endplates.
The cause is reduced pressure in the wingtip vortex trailing behind the wing. Reduced pressure causes reduced temperature and if ambient humidity is high enough the result is the condensation you observe.
As other answers state, the vapor trail from the edge of the wing (or from other sections of the wing in various configurations) is the most visible effect of the wingtip vortex, a phenomenon in aerodynamics of airfoils.
Succinctly put, along most of the wing, the air passing over the wing and the air passing under it are pretty cleanly separated by the wing, and when reintroduced, while there's significant turbulence (creating "induced drag") there's typically very little rotational quality to it; a sheet of air over the top of the wing meets a sheet of air from the bottom, where it mixes and settles.
However, at the wingtip, there's a stark border between air that isn't being separated by the wing and air that is. The air that passes over the top of the wingtip is typically directed downward as it leaves the wing, similar to the rest of the wing, and at the wingtip this downward force, in addition to the pressure differential at the wingtip itself (the higher pressure under the wing seeks to equalize with the lower pressure above it by moving around the wingtip), induces a rotation in the air column behind the wingtip; clockwise on the left wing, counter-clock on the right:
Spinning air creates a volume of lower pressure in the center of the column, like the eye of a hurricane, and when air conditions are just right, this reduction in pressure in the center of the vortex can cause a contrail as the humidity in the air condenses into fog. The greater the force of lift produced by the wing, the more forceful and thus larger these vortices can be; wingtip vortices behind large commercial airliners can extend for miles behind the airliner, posing problems for smaller craft in their wake especially in airport traffic patterns. Smaller craft, especially fighter jets, can produce similar contrails in high-G maneuvers as the higher angle of attack of the wing produces a severe pressure differential just above the wingtips, increasing the vortex effect. Modern 4.5 and 5th-generation jets with stealth in mind are designed to avoid this as it's a visual cue to the jet's location.
The effect can be even more pronounced at the edges of deployed flaps, closer to the aircraft fuselage; here the flaps are inducing even more of a downward force on the air, and there's a similar discontinuity between the corner of the flap and the relatively straight wing next to it. Flaps are also typically deployed at lower altitudes where the air's thicker, warmer and can hold more moisture, so the atmospheric conditions for producing a vapor trail are more easily met: