Monitor calibration is a very important task, but it's one that a lot of people either don't get round to or don't think they need. If you have ever printed a photograph and had the colors look different from the screen to the print, then your calibration is off.
The Spyder3 Express is available from Amazon (pre-order in the UK) and retails for £77/$85. There's no excuse to not calibrate when the Spyder3 Express is so inexpensive. If you have more than one monitor, you need to have one of these even more. If like me, you sometimes have to start your edits on a laptop and finish them on your desktop computer, calibration will stop all those annoying readjustments that you need to make after you transfer your images. I'll have a full review on the Spyder3 Express in a week or two when I have a test model.