Simply this: if anyone you know is planning to adopt a pet for Christmas, please encourage them to adopt a shelter pet. I know you are just one person, and how much difference can that really make, but trust me, it does.
I have a college friend who works here: http://www.humanesociety.org/ and while I don't agree with some of their more far-fetched political lobbying, I do believe in the work they do to stop tethering, puppy mills, and other forms of animal abuse and cruelty. Feel free to donate to them, or google your local chapter of the SPCA and donate.
My local SPCA (check out this cutie who I daydream about bringing home to keep my high-maintenance, neurotic 4 year old mutt company) has a Black Dog and Black Cat club. Until I started donating to pet causes, I had no idea that black dogs and cats were the least likely to be adopted out. If I was a good, prepared Blogger, I would include a picture of my girl here, because one look (or cuddle) with her and you would be wanting your own black shelter dog today.
(Edit: I couldn't resist throwing her on here after I got home from work.)
I know some people are all about their AKC-certified yappy lapdogs or whatever, but I can honestly tell you that any pet you get from a pet "store" or flea-market stand is likely to come from owners who only care about whoring out their mature animals to make money. That is just not right to me. I have seen enough of what happens with pet overpopulation to ensure that any pet I ever adopt will be a rescue. Use caution when answering craigslist or newspaper ads for "puppies" as well. While a lot of those are legitimate, make sure you visit where the pets are actually housed to ensure this is not someone who consistently allows their dog to get pregnant so that they can sell the litter. If you have to pay anything for a dog or cat, it should only be the adoption fee from a certified rescue shelter to cover their vet bills and operating costs. NEVER pay money to someone selling a littler of puppies at a roadside stand.
And finally, as Bob Barker always said, once you bring that lovable lovely home, PLEASE spay and neuter your pets!