Well there's the popular petzlife, but Im still not convinced since there have been users here who say they don't see any difference. However, there is something similar called plaq clnz gel that claim they are the only real gel product that cleans tartar without using a toothbrush.
They currently giving out samples and I got my little bottle sample last week and have started to use it. It's a little early but I think I see some of the brown/yellow stains shows signs of lightening. I will know more in the weeks to come so I will post back. Heck it's free so why not give it try...
PlaqClnz the New Oral Cleansing System for Dogs and Cats.