Permabit has a really interesting appliance, and hopefully they can pull it together and get their message out there because they have a robust product that's been around for a few years. It's probably more mature than a number of the products out there.
The other product I'm really fascinated by is the NEC Hydrastor. It just had its first product release at Storage Decisions in New York City earlier this year, and I've talked to a couple customers that had good stories to share about it. It's got a really solid architecture, but it's so new there hasn't been time for users to start using and evaluating it so NEC can work any kinks out that emerge and get it ready for the enterprise.
Check out the entire Data Deduplication FAQ.