I own a small rotery laser (about 4"x4"x4") If you put it on the floor so the beam runs across the floor, ceiling and walls, you can move it around until you find the route with the fewest obstrutions, once you find your spot you can put all your supports up and if the ceiling has different levels the light still hits where the string would be below.
Using it the other way you set hights for whatever you are doing.
Here is a link to the one I have about $400.00 http://www.laserlevels.net/momentum.html
To be honest mine is too dim most times you need to where glass that come with it.
The Shop has a Dewalt kit which is much brighter and can be used for elevations outside with a receiver.
Here is a link to the Dewalt Kit about $1,100.00 http://www.dewalt.com/us/products/tool_detail.asp?productID=2901
The more you use it the more you will like it, and the more things you will think of to use it for.
[This message has been edited by iwire (edited 03-11-2003).]