You can get hardware colorimeters that cost anywhere from £60 to £250+, with the more expensive ones being more sophisticated and arguably better at calibrating. TFTCentral for example uses the
LaCie Blue Eye Pro which retails for around £265.
The Spyder3Pro (~£100) or
Spyder3Elite (~£150) and the
Eye-One (i1) Display 2 (~£150) are probably the most popular. Here's a table showing the differences between the Spyder3 models (from
this review):
On the cheaper end of the scale you can get the Pantone Huey (~£60) or Huey PRO (~£85) and the Spyder3Express (~£70), whilst these are cheap, they're also quite limited and I believe some have compatibility problems with Windows 7, particularly 64bit.
Whether you actually need a hardware colorimeter is something you think about. Unless you absolutely need accurate colours for colour critical work, then I'd say you might be better off saving some money and just calibrating by eye.
I mainly use the
Lagom tests (and sometimes
this page) to calibrate my monitor by eye as well as these 2 images (amongst others):
Calibrating by eye will never get you the perfect calibration, but it's certainly cheaper than buying a hardware calibrator.