I excerpted the link above, but the long and short of it is that it has been well-documented that many early sightings were actually early AWACS radar planes and stealth aircraft. I firmly maintain that this is still the case. AWACS planes and stealth surveillance jets fly all over the world. In fact, many, many recent reports are of the "V-shaped craft" variety.
I also theorized earlier in the thread that the Harrier jump-jet with its ability to hover could be possibly be mistaken for a UFO by those with an active imagination. And the fact still remains that nearly all "sightings" take place near military bases. And this is easy enough to explain, because it's only when these aircraft are doing their approaches for landing that they are at a low enough altitude to be seen by the naked eye.
AWACS Airborne Warning and Control System