I remember thinking how cool it would be to write a program that would use my cell phone's camera to scan a book bar code then automatically add that book to a digital library or shopping list.
While Microsoft Tag wont let you scan traditional UPC bar codes, it does let you create your own bar code that will send a user to a website, place a phone call, add a contact or just show regular text. The main thing I see this being useful for is posters with a URL. Imagine seeing a concert poster and wanting to buy tickets online, where all you have to do is point your phone at the poster and it pulls up the ticket purchase page.
Pretty cool stuff, but of course it all depends on people adopting the technology and printing these bar codes on their products.
You sign up for the beta by visiting tags.microsoft.com. Just for fun, here's a tag for this Blog.