Barcode scanner apps are particulary useful on a mobile device, expecially if they REALLY work. Here's the best you can get for Windows Phone 7 at the moment: PhraseMeme Scanner by Gregarious Gaming.

This is a multi-format scanner that supports a handful of barcodes, including UPC, EAN, QR, and a lot more. To scan a tag you just have to take a picture of it from the app main screen. If all goes well (and it usually does), you can then send results to some online services like Windows Phone Search, Google Product Search, eBay, Amazon, Half.com OR share them in a variety of ways (SMS, E-Mail, social networks).

The interface is pretty basic, buy also very functional and user-friendly; the app is as good as it can be, with the tools that Microsoft provides right now to third party developers. One thing that would improve accuracy would be "realtime scanning", which will hopefully come in future OS upgrades. Right now, even the official app for scanning MS Tag forces you to take a pic of the Tag, instead of accessing the camera video stream directly.

However, PhraseMeme provides a handy guide and some tips to take a good pic and increase accuracy. Also, if you have issues with scanning tags, you can send feedback and details about the issue to the developers directly from the app menu, and have them looked into as soon as possible.

A fully featured trial version of PhraseMeme Scanner (with nag screen) is available on the Marketplace, and you can buy the full version for USD1.99, or the equivalent for your country. If you're looking for a barcode scanner app (and you don't care about it having an attractive icon) look no further.