Paint them black then write labels in white paint marker to look like chalk on blackboard.
make a wind chime
Paint them and sell them with phone #'s of owners
You could glue/decoupage decorative paper and little things like postage stamps on them and make a charm bracelet or pretty pendant out of them!
Best of all, turn them into your local town, village, city clerk.
If they're total blanks, you can get metal stamps and stamp them yourself.
Donate them to a free or discount spay & neuter clinic?
I used dog tags to identify the oils and vinegars in my kitchen. I store the liquids in vintage liquor decanters. Then I had the tags engraved with each name (e.g. "walnut oil" and "balsamic vinegar") and used pieces of bead chain to loop the identifying tag over the neck of each decanter.
I made a wind chime using things that hold dear memories. Including pet tags, our first house key, an old jewelry box key, baby spoon and a child's ceramic tea set (the cups).
I have three dogs. I keep all of thier expired tags. I'm not quite sure what I'm going to do with them, but I put them with my beads and jewerly craft stuff thinking they might work into a type of jewerly item, "dog tags". I lost the pull on one of my coats and clipped a dog tag onto it as a pull. In fact, it's my Sheltie coat. People notice it, too, and comment about it. They could be used as key rings. My dad gave me a bunch of stuff and in it were old dog tags.