I've got a little torpedo level that shoots 3 directions. It's become invaluable. Like iwire said, they become more useful the more you use them. (Keep an eye on these. They seem to be a magnet for thieves. This is the 3rd one I've bought)...S
I've used the the ones that shoot vertical up and down, plus 90 degree horizontal lines. their great for laying fixture marks on the floor and transposing the marks to the ceiling. The rotating ones are good for getting conduit runs straight
I have a 18" level that projects either a spot or a beam of light and the Black and Decker laser stud finder. It self levels so you just hang it on a pin and turn it on. The 18" level is also nice to beam where you think your conduit run needs to go. It will show if any obstructions are in the way.
Keep an eye on the stated accuracy, the new Laser-trac from Sears states 1/4" in 30 foot.
I've got a small unit about the size of a tape measure. It has 2 push-pins that you just stick into a stud or drywall. It works kind of like an electronic chalk line and is handy for lines at angles like a stairway or vaulted ceiling.