For measuring absolute level over a large distance, buy a water level:

http://www.leevalley.com/wood/page.a...ren cy=2&SID=

Use the water level to mark some perfectly level endpoints,
and then use the laser to draw a perfectly straight line between
those two points. You can then use a tape measure to see how far
your walls or floors are out of true.

Here's a cool water level that gives you a bit of sophistication.
Maybe not laser sophistication, but easier to use and a whole lot more
accurate.

http://www.zircon.com/SellPages/Leve...WL25/WL25.html

I used this grading my patio (pavers) and I was amazed at how accurate
the tool was and how perfectly graded the site ended up being. I
improvised a bit, taping the working end of the tube onto a stick
(making sure it could still slide a little, inside the tape). I
marked level on the stick and then marked it in quarter inch
increments up the stick, to easily establish and check grade. For
every foot, I would line the water level a quarter inch higher.
Worked quick, easy and accurate.

It won't work for sagging walls, but certainly helps for measuring
level (not plumb) over longer distances.