Greenies but there seems to be a lot of bad press regarding them being dangerous (although they've since changed their formula). I decided to try it and bought a small bag of them for Meiby. I gave her one and monitored her the whole time. Even though I was watching closely, she still gulped down a huge chunk of it and I was worried for a little bit. It turned out fine but I haven't given her any more of them.
Last week at Target I came across a new product: 3M Dental Treats. They have 3 "prongs" to keep your dog from swallowing them too quickly. The ones I bought were white though (not brown like the one pictured in the link) and smelled minty. I gave one to Meiby and she loved it, ate it slowly while holding on to the prongs with her little raccoon claws, and had nice breath when she was finished! I definitely recommend these over Greenies. She really takes her time and chews on them, getting the full benefit of plaque removal that's intended, and they definitely smell better (not that Greenies smell bad, they just don't smell much like anything.) They are a bit expensive but I certainly don't give her two every day as the package suggests. I think one a week is plenty. (There's another good review of these at epinions.) Anyone else tried these?