Regarding UFO's as alien space/air crafts:
There is a [size=medium]LOT[/size] of space out there, I could give you numbers on how big the universe is, how many stars and galaxies there are, about how many stars have planets, but the numbers will be just too big for anyone to truly comprehend. But to think we are the only ones in this massive universe is ridiculous.
HOWEVER, going back to my original point about space (unoccupied/empty)-- there is simply WAY TOO MUCH space in-between stars and galaxies, and for any intelligent alien race to travel it just to see our little planet is again, ridiculous.
The nearest star to us is about 4 light years away. What is a light year? A light year is how long it takes LIGHT to travel from this star to our planet. The nearest galaxy is about 2.5 MILLION light years away.
And for the record, no matter how fast you can build a space craft, NO ONE can travel faster than the speed of light. C is like the speed limit for everything. So assuming these aliens (which probably wouldn't look too far different from us... but that's getting into a whole different discussion), can travel at near the speed of light-- they would be dead way before they ever got to our little blue planet.
The only way I can see it happen is with the use of worm holes, but that is all theory and science fiction at this point.
Are there other living organisms and intelligent life out there? Most probably. Has anyone on Earth been anally probed or even seen them-- or will ever see them? Highly unlikely.
Pick up some books or take a class and enlighten yourself about space and the universe before you start to believe this UFO nonsense. I'd also be happy to elaborate and go into this further if you'd like.
But I guess if UFOs are real then they would have to be made by us, IE the military.