Hard to explain what this is - it makes your TV or monitor look better by setting it up more accurately. Currently US$70 reduced (until the end of June).
It connects to a computer and 'reads' the settings of your display (TV, monitor etc) to help you configure the displays settings to be as accurate as possible. This one has hit a new price point for its quality.
TV's are rarely set as good as they can be out of the box. They might have overly dark shadows, or too red flesh tones. You can adjust that to a certain extent with the normal adjustments (Brightness, Contrast, etc), but to get the best out of it you need to have it calibrated. You can get a tech in (no idea what that costs, but I'd guess at least a few hundred) or with one of these and access to the service menu of your TV (usually a combination of remote buttons gets you into it) you can do it yourself.
If you get the version with AutoCalibration ($671) and the DVDO iScan Duo I posted ($970) you can automatically run calibration (as well as the other benefits of the DVDO). The drawback to this is it's the DVDO that is being adjusted, not the TV, so it's a one display device solution (as far as the automation goes). You could still manually calibrate other displays with the X-Rite.
http://www.xrite.com/product_overview.aspx?ID=1454
http://www.dtvforum.info/index.php?showtopic=90558 (forum discussion of this and the DVDO iScan Duo)
no deal
amazon is selling @ US$300
p.s: clicked the wrong one :(