I've seen several manufacturers, like challenge, offering what they call an 'Open Tubular' tire. How is this different from a standard clincher tire?
An "open tubular" is basically a tubular tire with clincher beads instead of being sewn. They use the same materials as the sewups, so in theory should offer some of the same advantages as tubular tires.
However, most of the pro's of a tubular (lighter weight, less rolling resistance, tire will stay on when flat) are voided because they've turned a tubular into a clincher. Plus, there are many modern clincher tires that have just as little or even less rolling resistance than a tubular (in some specific cases).