Solar heating causes uneven heating of the earth's terrain due to the varying Specific Heat Capacity of the exposed surfaces. This in turn causes convection currents in the atmosphere (ie. wind).
Wind farms harness the kinetic energy of the wind using turbine blades which convert the linear motion of the wind into rotary motion which runs a generator, and outputs electricity.
akf2000 mentions the infeasibility of widespread implementation of wind farms due to their obtrusive nature. Although the energy supplied is from a completely renewable source, the efficiency of these turbines is such that any appreciable power output takes up vast amounts of land area. Land area which becomes rather unattractive.
Thanks to my incredibly slow reading of New Scientist (UK Edition -- are there any others?) I only very recently read the 23 September 2000 issue (No.2257). In it there was an intriguing article titled "Reach for the Sky" (p.36), talking about Bryan Roberts' experimentation with alternatives.
A "gyromill" is "a cross between a helicopter and a kite" which flies at an altitude of 4km, and is tethered to the ground with steel cables (which double as means to transfer the generated electricity down to the ground).
The idea behind the gyromills is that they catch the jet streams in the atmosphere and use the energy from them to firstly stay afloat, and secondly generate electricity. Energy available in the jet streams is up to 17 kilowatts per square meter -- compared to an average of 0.4 at ground level!
Additionally the "wind farms" consisting of these gyromills will be very unobtrusive, if at all noticable!
Unfortunately I was not able to track down this article on newscientist.com as full contents are only available to subscribers! If anyone manages to find the original story somewhere please /msg me!