I use PANTONE/GRETAG EyeOne Display 2. I paid less than US$ 200.
It works with CRTs, LCDs and laptop screens. It's a USB device. I can use it for measuring the ambient light level, but that's almost irrelevant. It allows me to leave the brightness of my monitor at a level that is comfortable for me (which is still within standard 75 to 100 candles). The software allows for an easy automatic calibration, in which case I don't need to touch any controls on the monitor, or for an "expert" input. At the end of the process it asks for naming and saving a new profile.
It works just fine for me, but almost as soon as I bought it I started thinking about buying a "real" calibration tool that would allow me to manage the color all the way from a camera or a scanner to a monitor to a print. Those devices usually are in the US$ 1,000 to 1,500 price range. They allow for the highest degree of accuracy and for additional "hand-tweaking" of a profile.
If anybody is interested in a professional opinion about calibration tools (and monitors, too), I suggest checking out this web site: ShootSmarter.com
. I wish I had an access to that info before I chose a half-baked solution...