Too often in science fiction we see laser beams being shot out of cannon like weapons.
Really though, how would they look? I guess it depends on the type of laser. But from what I've been told, non-visible lasers give the most bang for your buck. I realize that no laser is visible except for when they hit the target, but there are also the non-visible kind from what I've been told (X-ray lasers).
With visible lasers, I have read that the larger the initial aperture of the beams (the glass head that fires it), the more range you can get out of the beam before it diffracts.
Alternately, if you wanted to fudge lasers you could get a giant mirror and make an epic death ray. Basically a huge magnifying glass that burns stuff via the sun, like humans do with ants.
Of course that would be restricted to solar areas only.